REFllANGIBILITY. 



guifliable. Tlie reafon of this indiftinftnefs is, that rays of 

 every kind oomiiig from all parts of tlic fun's difc, an entire 

 image of the- fun is projeftcd on the paper, coiiiillincr of a 

 circle of each particular colour ; and as the rays differ in 

 kind by infinitefimal degrees, from the extreme red to the 

 extreme violet, there mull, in faiS, be thoufands of thefe 

 circles in tlie fame oblong image, the centres of which are 

 infinitely near to one another ; io that the ligiit is intimately 

 mixed, efpecially in the middle of the image, where it is the 

 brighteft. 



If thefe circles, as he obfervcs, whilft their centres keep 

 their diftances and pofitions, conld be made lefs in diameter, 

 their interfering one with another, and confequently the 

 mixture of heterogeneous rays, would be proportionably 

 diminifhed. Thus, in PT, (fig. lo.) the circles of which 

 the folar image confifts, expand into one another ; but in 

 the fame figure, / /, being compofcd of Icfs circles, but 

 having their centres at the fame diilance as the- former, do 

 not extend into one another, the mixture being diminifhed 

 in proportion to the diameters of the cu'cles. 



Now thefe circles woidd be diminifhed, if, without the- 

 room, at a great diftance from the prifm, towards the fun, 

 fome opaque body was placed, having a round hole in the 

 middle of it, to intercept all the fun's light, excepting fo 

 much as, coming from the middle of its difc, could pafs 

 through that hole to the prifm ; for fo the feparate circles 

 would no longer anfwer to the whole difc of the iun, but 

 only to that part of it wliich can be feen from the prifm, 

 through tliat hole. But that thefe circles may anfwer more 

 diftinftly to the hole, a lens is to be placed by the prifm, 

 to caft. the image of the hole, that is, of each feparate circle, 

 diilinftly upon the paper ; and if this be done, it will not 

 be neccdary to place that hole very far off, not even beyond 

 the window. Inftead, therefore, of that hole, he made 

 ufe of the hole in his window-fhuttcr, in the following 

 manner. 



At about IO or 12 feet from the window, he placed a 

 lens, by which the image of the hole might be dillinttly 

 caft upon a fheot of white paper, at the diftance of 6, 8, 10, 

 or 12 feet from the lens. Then, immediately after the 

 lens, he placed a prifm, by which the refraftcd light might 

 be thrown upwards, or fideways ; and he moved the paper 

 that received it, either towards the prifm, or from it, till 

 he found the exaft diftance at which the fides oi the image 

 appeared moil diiHnft. By this means the circular images 

 of the hole were terminated moft diftinftly, without any 

 penumbra, and therefore extended into one another the lealt 

 that they could ; and confequently the mixture of the he- 

 terogeneous rays was the leaft of all. And by ufing a 

 greater or lefs hole in the window-fhutter, he made the cir- 

 cular images greater or lefs at pleafure, and thereby the 

 mixture of rays in the oblong image was as much or as little 

 as he chofe. He fometimes made the breadth of the image 

 40 times, and fometimes 60 or 70 times lels than its length. 

 In this manner, he fays, light is made fufficicntly fimple, 

 and homogeneal, for trying any of his experiments about 

 fimple light ; for that the heterogeneous rays in this light 

 are fo few, as hardly to be perceived, excepting, perhaps, 

 in the indigo and violet, which, being dark colours, do eafily 

 fuffer a fenfible allay by that httle fcattering light, which 

 ufed to be refrafted irregularly by the inequalities of the 

 prifm. The whole procefs of this experiment is fo evident, 

 by infpcftion oi Jig. 1 1, that it needs no particular illuilra- 

 tion. 



Inftead of a cjircular hole, our author recommends a hole 

 fliaped like a long parallelogram, v.'ith its length parallel to 

 fhe priftn. For if this hole be an inch or two long, and 



8 



but a loth or 20th part of an inch broad, or narrower, tl/ 

 light of the image will be as fimple as before, or more 

 fimple, and the image will become much broader, and 

 therefore more fit for thefe experiments. 



Or, inftead of this hole, another may be formed of a tri- 

 angle of equal fides, whofe bafe may be about the loth 

 part of an inch, and its height an inch or more. For, by 

 this means, if the axis of the prifm be parallel to the per- 

 pendicular of the triangle, the image ^/ {f'S- ■^•) ^'"'^ "''^' 

 be formed of equicrural triangles, ag, bk, ci, dk, el, fm, 

 &c. and innumerable other intermediate ones, anfwering to 

 the triangular hole in (hape and bignefs, and lying one after 

 another in a continual leries, between two parallel lines, af, 

 g m. Thefe triangles are a little intermingled at their bafes, 

 but not at their vertices ; and therefore the light on thc- 

 brighter fide, af, of the image, where the bafcs of the tri- 

 angles are, is a little compounded, but on the darker fidi-, 

 g m, it is altogether uncompounded ; and in all places be- 

 tween the fides, the compofition is proportionable to the 

 diftances of the places from that obfcurtr fide gm; and 

 having an image of fuch a compofition, we may try experi- 

 ments either in its ftronger and lefs fimple light, near the 

 fide af; or in its weaker and more fimple light, near the 

 other fide g m, ar. fliall feem moft convenient. 



In making thefe experiments, he advites, that the cham- 

 ber be made very dark, that the lens be very good, being 

 made of glafs free from bubbles and veins, the fides ef the 

 prifm truly plane, and its polifti elaborate, with an angle of 

 about 65 or 70 degrees ; and the edges of the prifm and 

 lens, as far as they make any irregular refraftion, fliould be 

 covered with black paper glued on them. He alfo ob- 

 ferves, that all the ufelefs light fliould be intercepted with 

 black paper, or other black obftacles. It being difficult 

 to get glafs prifms fit for thefe nice experiments, he fome- 

 times ufed prifmatic veffeb made with pieces of broken 

 looking-glailcs, and filled with rain-water ; and to increafe 

 the rcfradlion, he fometimes ftrongly impregnated the water 

 with faccharum faturni. 



When he had, by this means, got the fides of the co- 

 loured image, as A F, G M, [Jig. 13.) diftinftly defined, 

 he delineated on paper the outlines of it F A P G M T, and 

 held the paper fo that the image might fall on this figure, 

 and coincide v;ith it exaftly ; whilit an afTillant, whofe eyes 

 could diflinguifh colours better than his own, did, by right 

 lines drawn crofs the image, mark the confines of each co- 

 lour ; and this operation being frequently repeated, both on 

 the fame and on different papers, he found that the ob- 

 fervations agreed well enough with one another, and that 

 the fides, M G and F A, were by this means divided like a 

 mufical chord; fo that if GM were produced to X, mak- 

 ing M X equal to G M, and if G X, IX, iX, iX, eX, 

 gX, aX, MX, were in proportion to one another, as the 

 numbers i, 4, ^, 4, -2, -f, -pV, and -i, and fo reprefented 

 the chords of the key, and of a tone, a third minor, a 

 fourth, a fifth, a fixth major, a feventh, and an eighth 

 above that key. And the intervals M a, ag, g e, eh, hi, 

 i I, and / G, were the fpaces which the feveral colours, red, 

 orange, yellow, green, blue, indigo, and violet, took up. 



Now thefe intervals or fpaces, fubtending the differences 

 of the refraftions of the rays, going to the limit of thofe 

 colours, that is to the points M, a, g, e, h, i, I, G, may, 

 without any fenfible error, be accounted proportional to the 

 differences of the fines of refrailion of thofe rays, having 

 one common fine of incidence ; and, therefore, fince tfte 

 common fine of incidence of the moil and leail refrangible 

 rays, out of glafs into air, was found in proportion to their 

 fines of refraftion as 50 to 77 and y&j if the difference be- 

 tween 



