32 ALEXANDEE JOHNSON 



actions ' for 1802, p. 378, where he says : " If a beam of daylight be admitted into a dark 

 room by a crevice one-twentieth of an inch broad, and received by the eye at a distance of ten 

 or twelve feet through a prism of flint glass free from veins " (italicized by Wollaston), 

 " held near the eyes, the beam is seen to be separated into the four following colours only, 

 red, yellowish-green, blue and violet." In a diagram accompanying the paper he notes 

 the lines, four of which he considers as boundaries of the colours. They are six in all. 

 Of two of them he attempts no explanation. He changed the materials of the prism, but 

 found no alteration in the lines while he used solar light. But using candle light and 

 the electric light he found the appearances, which, says he, " I cannot undertake to 

 explain," different. 



That Newton did not see the dark lines is very remarkable when we consider the 

 great number and variety of his experiments. Among the causes assigned for this it is 

 said, or implied, that Newton always received the spectrum on a screen, whereas Wollas- 

 ton saw the lines by simply looking through the prism. But Newton mentions that he 

 looked through the prism also (Prop. II, Bk. I, p. 22), but it was at the round hole about 

 a quarter of an inch in diameter. If he had been using the slit on this occasion he might 

 have anticipated Wollaston. The other chief cause assigned is that he never used a slit 

 or lens, and did not understand the advantages of them. But, on the contrary, we see 

 that Newton was perfectly aware of the advantages of a narrow slit. In his eleventh 

 experiment he uses a circular hole one tenth of an inch in diameter. After this he men- 

 tions a slit one-tenth of an inch broad, then oue one-twentieth of an inch, then " nar- 

 rower," and, he remarks, " the light will be as simple as before or simpler, and the image 

 will become much broader, and therefore more fit to have experiments tried in its light 

 than before." But he goes farther still in comparing the eflects of different breadths of 

 the slit ; for in taking the long, narrow, isosceles triangular opening he makes its base 

 the same as the diameter of the circular hole above referred to, namely, one-tenth of an 

 inch, and its perpendicular height being an inch or more, the width of this slit tapers 

 off from one-tenth of an inch to nothing. 



II. Experimental. 



While getting this paper ready for the printer I took some opportunities for repeating 

 the experiments in which Newton used the slit and lens, as closely as possible in Newton's 

 own manner, not expecting much from them as regards the dark lines, as I had never seen 

 any hint given that the lines might be seen in this way, yet thinking that, with a pre- 

 vious knowledge of their existence, they would be visible on careful inspection, and that 

 in the experiments as performed by Newton they might have been overlooked, because 

 of his entrusting the division of the colours (in seeking for which Wollaston discovered 

 these lines) chiefly to an assistant, in whose eyes he had more confidence than in his 

 own. 



Neivton's me/hod. — Newton's method, as may be seen by a comparison of different 

 places in the " Opticks" and also by the instance he quotes in Experiment 11, was to place 

 the lens at or about double its focal length from the aperture, by which means an image 

 of the same size as the aperture might be received on a white paper screen about the same 

 distance beyond the lens, then to put the prism immediately behind the lens, receive the 



