NEWTON AND THE COLOURS OF THE SPECTRUM 255 



reached this stage, he points out that the opaque body is not 

 necessary ; the hole may be quite as well made in the shutter 

 itself and focussed by a lens on the other side of the room. 

 So he arrives at the ordinary arrangement in use to-day. 



We find this arrangement described in expt. n (p. 47) and 

 represented in fig. 24. F is a round hole in the shutter, MN 



is a lens at a distance of about 10 or 12 ft. from the window, 

 which throws an image of the hole on a sheet of white paper at 

 I, which is at a distance varying between 6 and 12 ft. from 

 the lens, according to the lens used. The introduction of the 

 prism causes the formation of the spectrum pt. The diagram 

 shows a circular hole at F, but on p. 49 we find : 



" Yet instead of the circular hole F, it is better to substitute 

 an oblong hole shaped like a long Parallelogram with its length 

 Parallel to the Prism ABC. For if this hole be an Inch or two 

 long, and but a tenth or twentieth part of an Inch broad or 

 narrower ; the Light of the Image pt 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." 



The use of a slit is again described on p. 81 (expt. 1, Bk. I. 

 Part 2), this time without a lens at all. 



We have thus the modern arrangement of slit, lens, and 

 prism, and the question arises, why did Newton not discover 

 the Fraunhofer lines ? With a slit ^ in. broad and a spectrum 

 8 in. long they should be visible when looked at from a suitable 

 distance, if a modern prism were used. Newton's failure 

 seems partly to have been due to his prisms. He apparently 

 polished them himself, and the glass was of poor optical quality ; 

 one prism he mentions as having " Veins, running along within 



