254 SCIENCE PROGRESS 



and a large part of the first book of his Opticks is devoted to 

 proving what is a commonplace now, namely, that the white 

 light of the sun was decomposed by the prism into light of 

 different colours, and that each of these colours had its own 

 definite refrangibility. The prisms he used had a good dis- 

 persion and gave a spectrum of a length of 2°, and on pp. 59- 

 60 1 he describes a determination of the indices of refraction 

 of two of them by a quadrant (an anticipation of Hadley's 

 sextant, but of somewhat different design), which gave results 

 4x||- and 4111- or in round numbers !£. 



With the arrangement shown in fig. 13 owing to the dia- 

 meter of the pinhole being J in. the spectrum would not be 

 sharp, but would have a penumbra | in. deep round its 

 boundary. Nevertheless on account of its simplicity this 

 arrangement seems to have been used oftener than any other. 

 In expt. 5, p. 28, however, we find Newton taking steps to 

 remove the penumbra, which he did by placing a lens at the 

 hole in the shutter. The dark room then acted like an ordinary 

 photographic camera, the lens throwing a sharp image of the 

 sun on the opposite wall. 



Even with a lens at the hole the spectrum is not pure, for 

 every colour forms its own image of the sun. Consequently we 

 have a series of coloured discs, red, orange, yellow, green, blue, 

 violet, all with their centres at different points on the same 

 straight line and all overlapping. The overlapping is greatest 

 on the line of centres and least at the edges of the spectrum. 

 The edges are, of course, defined by the two common tangents 

 to all the discs. The spectrum is thus very impure in the 

 middle, but increases in purity towards the edges. On pp. 46 

 and 47 we find Newton discussing the impurity of his spectra. 

 He ascribes it to the diameter of the sun not being small 

 enough, and suggests placing an opaque body with a round 

 hole in the middle of it without doors, at a great distance from 

 the prism in the direction of the sun. The coloured images 

 then produced inside the darkened room would not correspond 

 to the whole disc of the sun, but only to that portion of it seen 

 through the round hole of the opaque body. He suggests 

 using a lens with this arrangement, the lens to be placed close 

 to the prism and to throw a sharp image of the round hole in the 

 opaque body on the wall of the room. And then, having 



1 The references are to the first edition. 



