All to give a bit of perspective in a historical sense, it's not to say Boscovich is right but as so little is to be found
on the net about him, and he did have an influence on the progress of science, it seems fair to post it here too
Perhaps later some bits by Maxwell and Faraday about light
471. So much on the subject of fire; now I will make a few remarks about light,which is given off by fire, & which, when present in sufficient quantity, excites fire. It is possible that light may be a sort of very tenuous effluvium, or a kind of vapour forced out by the vigorous igneous fermentation. Indeed, in my judgement, there are very strong arguments in favour of this hypothesis, as opposed to all other hypotheses, such as that of waves. On the hypothesis of waves, Huygens once tried to explain all the phenomena of light ; & the most noted of the geometers of our age have tried to revive this theory, which had been buried with Huygens; but, as I think, unsuccessfully (r).
For, they have explained, & even then poorly enough, a very few of the properties of light, leaving the rest untouched ; & indeed I consider that such properties can not be explained in any way by this hypothesis of waves, & my opinion is that some of them are altogether contrary to it. But this is not the right place to impugn this theory ; indeed I have already, more than once, presented my view in other places. It is really marvellous how excellently, on the hypothesis of emanating effluvia, all the different properties of light are derived from my Theory in a straightforward way.
I gave a very full explanation of this in the second part of my dissertation, De Lumine ; & the principal points of this work I will touch upon here.
Meanwhile, I will just mention that the idea of effluent matter seems to be altogether reasonable; more especially from the fact that, in a very great agitation amongst particles, such as there is in the case of fire, there is always bound to be, in accordance with what we have seen in Art. 195, an abundance of particles flying off, just as we have evaporations in ebullition, effervescence & fermentation.
(r) When I wrote this, the Transactions of the Academy of Turin had not been published ; and even now, at the time of this reprint of my work, I have so far been unable to see what that excellent geometer La Grange has published on the subject.
472. The principal properties of light are : its constant emission, & the fact that the intensity is always the same from the same mass, such as from the Sun, or from the flame of the same candle ; its huge velocity, for it traverses a distance equal to twenty thousand times the semi-diameter of the Earth, which is about the distance of the Sun from the Earth, in an eighth of an hour ; the slight differences of velocity that exist in different rays, for it is proved from several indications that there is scarcely any difference for homogeneous light, if there is any at all ; its rectilinear propagation through a transparent medium everywhere equally dense, along with hindrance to progression through opaque media ; & this without any sensible hindrance due to impact with one another of rays having so many different directions, or any that prevents passage into the inner parts of transparent bodies, no matter how dense they may be ; reflection of part of the light at equal angles" at the surface of separation of two media, the part that is reflected being greater with regard to the whole amount of light, according as the obliquity of incidence is greater ; refraction of the other part at the same surface of separation, with the law of a constant ratio between the sines of the angle of incidence & the angle of refraction, the ratio being different for differently coloured rays, upon which depends the different refrangibility of the differently coloured rays ; dispersion, both in reflection & in refraction, of a very small part of the light in directions of every description whatever ; the alternation of propensity in any one ray, in one of which the light falling upon the surface of separation between two media of different nature is the more easily reflected & in the other is the more easily transmitted, which Newton calls ' fits ' of easier reflection & easier transmission, with intervals between these fits, after which the propensities mostly favouring reflection or refraction return, these intervals being equal in the same ray entering the same medium, & different for differently coloured rays, for different densities of the medium, for the different inclinations at which the ray enters the medium ; upon these fits & the different intervals between them for differently coloured rays depend all the phenomena of thin plates, & of natural colours, both variable & permanent, as well as the colours of thick plates, all of which have been discussed with considerable clearness by Fr. C. Benvenuti, a most careful writer of our Society, in his well-known dissertation, De Lumine. Last of all, we have that property, which is called diffraction, in which rays, passing near the edge of a body, are bent inwards, having a different colour & different refrangibility for different angles.
473. What pertains to emission has been already explained in Art. 199 & Art. 461 ; there also it was shown that, if the mass emitting the effluvia remained the same, then the amount emitted is practically the same in any given time.
Further, it may happen that the mass emitting the light is completely broken up, as takes place in sudden flashes of fire ; or it may be that this mass persists for a very long time. This to a very great extent depends on the size of the interval in which the oscillation due to fermentation takes place, & on the nature of the attractive arc at the end of that interval, by Art. 195.
Nay, if the Author of Nature had wished that a mass, agitated by the most vigorous fermentation even, should be quite irreducible by any finite force whatever, he could easily have accomplished this, as shown in Art. 460, by other asymptotic arcs with infinite areas, between the confines of which the fermenting mass would be situated.
By the aid of these arcs the mass could be so bound together, that it would not admit of the slightest dissolution ; & then by placing the material for emitting light further from the particles of the mass than the interval between those asymptotes, & within the distance corresponding to an attractive arc of huge but finite area ; from which we should have particles, one after the other, of light flying off.
Nor is there any difficulty from the usual argument that is raised in objection to this, that the mass of the Sun must be much diminished by such a large emission of light ; if we suppose indefinitely great componibility, & the solution of the problem, given in Art. 395.
For in any exceedingly small space there may be any huge number of points whatever ; & the whole mass of the light, which is diffused throughout & occupies such an immense volume, may, in the Sun or near the Sun, have occupied a space as small as ever one likes to assign ; so that the Sun, after the lapse of any number of thousands of centuries, will not therefore have decreased by even a finger's breadth.
It all depends on the ratio of the density of light to the density of the Sun, & this ratio can be any small ratio whatever. Indeed there are perfectly valid arguments for the immense tenuity of light, some of which I will give below.
474. Any velocity, no matter how great, can be obtained from sufficiently powerful repulsive arcs, if these occur after the last limit of oscillation within the confines of a very great attractive arc, as shown in Art. 104.
For if a particle goes off from here with no velocity, the square of the whole velocity is defined by the excess of all the repulsive areas over all the attractive, as was shown in Art. 178 ; &, as this excess can be of any amount whatever, the velocity can also be of any magnitude whatever.
Again, the difference of velocity for homogeneous particles is quite insensible, because particles of light of the same kind come to the end of their oscillation with velocities that are almost zero ; for those which, according to the Theory set forth in Art. 195, increase their oscillation gradually, arrive at the boundary limiting the mass at last, & then fly off.
Now, if, at the time they fly off, they should reach this boundary with a very great velocity, then it is certain that they would have reached it & flown off in a previous oscillation.
Further, in the same article, we have proved that a slight difference of velocity on entering a space, in which given forces continually accelerate the motion & generate a huge velocity, also induces a difference in the velocity generated that is very small even when compared with the slight difference in the initial velocity.
This we there prove from an argument derived from the nature of the square of a very large quantity compounded with the square of a quantity much less than it ; this gives a quantity differing from the first quantity by something much less than the small quantity of which the square was added. A sensible difference may be obtained, if what fly off are not simple points, but particles somewhat different from one another.
For the curve of forces, with which the mass acts upon such particles, can be somewhat different for those different particles ; & thus, the excess of the sum of the repulsive areas over the sum of the attractive may be somewhat different, & therefore the square of the velocity corresponding to this excess may be somewhat different.
In this way particles of homogeneous light will have velocities that are practically equal ; but particles of heterogeneous light may have velocities that are somewhat different ; as seems to be conclusively shown from observations of phenomena.
One thing remains to be noted in this connection, namely, that the curve of forces, with which the whole mass acts upon a particle placed already beyond the limit of the oscillation, when the points of the mass are changed on account of the oscillation, will be somewhat altered.
But since in a very large irregular agitation of the entire mass all the different positions of the points follow on after one another very quickly, the sum of all the forces will be practically the same, especially in the case of a particle stopping for some time at the beginning of its , flight ; which point it has reached, as we have said, with a velocity that is exceedingly small.
Thus, the velocity of homogeneous particles must on that account be practically the same, when they have reached the arc representing gravitation ; & a difference can only be obtained in heterogeneous particles owing to their structure. It is therefore clear from what source the very great velocity can come, & also the slight differences, if there are any.
475. That which relates to the rectilinear propagation through a transparent homogeneous medium, & the free motion, without hindrance, by particles either of the light or of the transparent medium, is quite easily explained in my Theory, whereas in other theories it begets a very great difficulty.
Also as regards hindrance to this motion, so long as the curve of forces has no asymptotic arc perpendicular to the axis besides the first, it has been shown, in Art. 362, that merely with a sufficiently great velocity there can be obtained an apparent compenetration of two substances ; & tenuity & homogeneity of space traversed will assist this to a very great extent.
Now, since, compared with perfectly indivisible & non-extended points of matter, there are an infinitely infinite number of points of space existing in the same plane, there is an infinitely infinite improbability that, for any instant of time chosen, the direction of motion of any one point of matter should be accurately directed towards any other point of matter ; & this improbability, when we consider the sum of all the instants contained in any given time, however long, still comes out simply infinite.
The number of points of light is indeed very large, not to say enormous, but in my Theory it is at least finite. These points at any chosen instant of time have an almost immeasurable number of directions of motion, but this number is finite in my Theory.
It is indeed true that, no matter where an eye is situated upon the well-nigh immeasurable surface of a sphere described about one of the remotest stars as centre, nay, or within that sphere, the star will be seen ; & thus, it is true that some particle of light must affect our eye.
But in my Theory, that does not come about because rays of light come to it accurately in every one of an absolute infinity of directions ; but because the pupil & the nerves of the eye do not form a single point, & the forces due to the points of a particle of light act at some distance away.
Hence, in any chosen time, no matter how long, there need not happen in my Theory any case, in which any point of light is directed exactly towards any other point either of light, or of any substance, so that it is bound to collide with it.
Hence, no point of light stays its motion, or deflects it, through collision or immediate impact.
476. This is indeed a common property of all bodies, that is, of bodies that approach one another.
In my Theory, they have no point directly colliding with any other point, For this reason I also stated, in the above-mentioned article, that, if no mutual forces were present, there is always bound to be an apparent compenetration of all bodies. Yet, from this article alone, it is utterly impossible that there ever can be real compenetration. Hence, forces extending over some distance will hinder the progressive motion.
If these forces are always equal in all directions, there would be no impediment to the motion, & it would necessarily be rectilinear owing to the force of inertia. Hence, nothing but a difference in the forces acting on a moving point can hinder it.
But if no infinite force occurs corresponding to any asymptotic arc after the first, all the forces are finite ; & so also the difference between the forces acting in different directions will be always finite.
Therefore, no matter how great the force may be, there is some finite velocity capable of overcoming it, without suffering any retardation, acceleration, or deviation amounting to any given magnitude, no matter how small. For, the forces require time to produce a new velocity, this being always proportional to the force & the time.
Hence, if there were a sufficiently great velocity, any substance would pass freely through any other substance, without any sensible hindrance, & without any sensible change in the situation of the points belonging to either substance, & without any destruction of the mutual connection between the points, or of cohesion.
There also I gave an illustration of an iron ball making its way freely through a group of magnets with a sufficiently great velocity ; & here also we saw that we owe what idea we have of impenetrability, in the case of forces that are everywhere finite, merely to the moderate nature of our velocities & forces ; for by their help alone we cannot impress a sufficiently great velocity, & freely pass through barrier-walls, or shut doors.
477. Now, this is the case, so long as there are no asymptotic arcs besides the first, to induce absolutely infinite forces ; but if, owing to such asymptotic arcs, the particles become incapable both of dissolution & penetration, as in Art. 362, then indeed by no velocity, however great, could one particle pass through another ; & the matter would be reduced to the same idea, as is held generally about the continuous extension of matter, Thus, in that case it would be necessary to diminish the size of the particles of light ; not indeed infinitely for I consider that that would be altogether impossible, just as also I think that there are no quantities infinitely small in themselves, and so determined without reference to any process of human thought ; nor is there anywhere in Nature any necessity for such quantities. But they must be so diminished that the direct collision of one particle with another in any chosen finite time will still be improbable, to any extent desired ; & this can be secured in every case by finite magnitudes. For suppose a plane area circumscribing each particle of light, & that this plane moves with the particle ; then the number of these planes in any given finite time, however long, will in every case be. finite, so long as the particles are distant from one another by any interval at all, no matter how small ; & thus, in any given finite time the mass, however luminous, can only emit a finite number of these particles. Further, any one of these planes will impinge, at their broadest parts, upon the middle of other particles of light distant from one another by a finite number of fits, in every case in a finite time ; for, this can only take place through a finite interval. The sum of such approaches pertaining to all the planes of the particles, finite in number, will also be finite, no matter how great the number may be. But we may so diminish the greatest diameters of the particles that the area of the plane, extended in all directions round to any given distance, however small, may bear to the greatest section of the particle a ratio greater, to any arbitrary extent, than that which is expressed by the huge but finite number of the approaches. Hence, the number of directions, by which all the planes pertaining to all the particles may pass without colliding with any particle, will be greater than the number of directions in which there may be collision, the ratio being one that is as immense as we please. And this will even be the case,if they should have to move in accordance with the law that one must not pass at a greater distance from the other than that interval which determines the very small space, to which it is supposed that the section of the particle bears a ratio of less inequality, no matter what the magnitude. There will nowhere be any need of the infinite in Nature ; a series of finites, extended indefinitely, will always give us something finite, which is large enough or small enough to satisfy any physical needs.
478. All that has been said with regard to particles referred to one another, the same will hold good for particles in reference to any bodies ; & especially if the bodies are formed, in accordance with my Theory, of particles distant from one another, & not bound together by a continuous connection, or possessing the truly continuous extension of the skin or wall offering a continuous infinite resistance, with which we dealt in Art. 362, 363. But really there is no necessity for such asymptotic arcs in my Theory ; in it also, by means of connections & forces of limits of any value however great, though not actually infinite, everything in Nature can be accomplished. If we are to adhere to the principle of induction, we are bound rather to think that there are no other asymptotic arcs in the curve which Nature follows. For, in the mighty interval between the stars & the smallest particles that are visible under the microscope, no connections of this kind occur, as is indicated by the continuous motion of the particles of light throughout the whole of these regions. Unless, perhaps, that first repulsive branch, & that last arc of the nature that pertains to gravity, are to be taken as a sign that there are also somewhere others like them, at distances which are less than microscopical, or greater than those within the range of the telescope. Besides, if all the forces are finite, and points of matter, in accordance with, my Theory, are perfectly simple & non-extended, it is far more easily understood why there can be this apparent compenetration, without any collision, & without any dissolution of the particles as they pass through one another.