56« ' On the Lifiex'ton, Rtjttxion, and Colours of Light, ' 



when I come to explain die reafon of the colours mnde by the refle£l!ng boJy, and the 

 manner of their formation. At prefent I fliall only caution thofe who may with to repeat 

 the above experiments, that the hole in the window-ftiut muft be fmall, the room quite 

 dark, the pin well polilhed, and the deflc, chart, ^'c. placed at a diftance from the pin not 

 greater than three feet, otherwife the images will be dilute and dim ; nor, on the other 

 hand, lefs than fix inches, otherwife they will be too (hort, and tiic colours not far enough 

 feparatcd one from another. 



My next objea of enquiry was the different degrees of reflexibility belonging to each 

 ray. It appears, not only from mathematical confiderations fufiiciently obvious, but alfo 

 from the experimdnts I have related, that though the different rays have, at the fame or 

 equal incidences, different angles of reflexion, yet each ray is conftant to itfelf in degree 

 of reflcxibility, and that its fine of reflexion bears always tlie fame ratio to its fine of inci- 

 dence. The queftion then is, what are the fines of reflexion of the different rays, the 

 fine of incidence being the fame to all .' 



Obf. IX. In fummer, at noon, when ihe fun's light was exceedingly (trong, and there 

 was not the veflige of a cloud in the fky, I produced an uncommonly fine fet of images 

 by fixing, at an inch from the fmall hole ^^th of an inch diameter, a pin ^'^th of an inth 

 diameter. One of the brighteft of thefe I let pafs through the deilc to the chart below at 

 2-!- feet from the pin, and the image was three inches from the fliadow, in a ftraight line. 

 I delineated it carefully by drawing two parallel lines for the fides, and marking the femi- 

 cjrcular ends. Then with the point of a fmall needle I marked the confines of the conti- 

 guous colours on one of the parallel fides, and afterwards drew acrofs the image parallel 

 lines. This operation I repeated with the fame and different images at many diftances from 

 the pin, and on different days, with various forts of pins and fizes of holes, &c. &c. and 

 all thefe repetitions were made before I once examined the refult of any one meafurement, 

 that I might be unprejudiced in trying the thing over again. I then compared the fketchea^ 

 of divided images which I thus obtained, and found fufficient reafon to conclude, that the 

 differences between the fines of reflexion in th« different rays were in the h.irmonical or- 

 der. For the divifions were nearly as ^, -^, 4i:> -rr> t't> t5> tV» which, when compound- 

 ed with the fcale, gave i, 44, -rr> |» it t> tt> I » and thefe are exaflly the change of the 

 notes in an oftave, obtained by taking the fums of the oftave, and a fecond major, a third 

 major, a fourth, a fifth, a fixth major, a feventh major, and an eighth, inftead of the 

 difference between a double oflave and a fecond major, a third major, and fo on. Thus 

 the fpeclrum by reflexion is divided exaftly as the fpe£trum by refra£lion, only that the 

 former is inverted, and the different rays have reflexlbilities, that are inverfcly as their re- 

 frangibilities. Having fettled this (I flatter myfelf) curious and important point, I pro- 

 ceeded next to enquire into the abfolute reflexibility of the extreme colours ; for if this be 

 known, the angle of incidence being given, the angle of reflexion of all the different rays 

 may be found. For obtaining a folution of this problem I made the following experi- 

 ment : 



Obf. X. The fun fliining ftrongly through the fmall hole in the window-flnit, and the 

 lays diverging into a cone whofe bafc fell on an horizontal chart 2 J feet from the hole, be- 

 tween the hole and chart I placed a fcreen which had a plate and fmall hole in it. The 

 rays palling through this fell on a fmall pin, fo placed that the images formed might 



3 be 



