26 DECOMPOSITION OF LIGHT* 



fereadthl-25th. more than 24 centimetres. (9. 36 inches) Its breadth was 

 ^ T of its length, when the aperture in the diaphragm was 

 6 millimetres. (2. 34 lines) Sometimes this was reduced one 

 half, and the contraction of the spectrum was proportional, 

 the breadth then not being more than -±5 '■> an d lastly, by 

 diminishing the aperture, it was reduced to 7 " T of the length. 

 Ciolq^rs bright As to the strength of the colours, they were vivid and 

 wnd vivid. bright as might be expected. The impression on the eye 



fanceit appear- was Slic ^i on account of the narrowness of the image, that 

 ed triangular, at a few decimetres ( two thirds of a foot) from the cloth 

 the poim. nmng ^e s P e ctrum appeared as two straight lines, forming a 

 small angle, the apex of which was at the red extremity, 

 and the base at the violet. On going nearer it appeared a 

 single line. It was the same, if the spectrum were examined 

 from a distance through a glass. This doubling or radiating 

 of the image depends on the conformation of the eye, and 

 is connected with some other phenomena, of which I may 

 hereafter give an account. 

 The line of de- The distinction of the colours, and their separation into 

 marcation seven classes, Mas likewise one of the objects of our en- 

 colour not quiry. Though the existence of this, distinction was per- 

 very precise, ceived, it must be confessed, that it Mas not easy to trace 

 all their divisions. I made some attempts to effect it, the 

 narrative of which I shall pass over for the present; merely 

 observing, that Newton did not make his division on a 

 More so in the spectrum thus narrowed, but on one much larger, obtained 

 common spec- in th( . ugua i way vv i t i, out a lons> ()pt. I, part 2, prob. I. 

 Theereen Lastly I shall observe, (hat the green colour in our spec- 



shortened ; trum did not extend quite to the middle of its length, M'hence 

 the blue and ** foU ow " c d, that the shades between the green and red 

 violet lengthen- were a little shortened, and those of the blue and violet 

 proporlionally elongated. These effects Mere owing no 

 from the nature doubt to the nature of the flint glass, of whack our prism 

 of the glass. was made. We had no opportunity of procuring common 

 glass free from streaks. Having tried a hollow prism, 

 formed of glasses joined together, and idled with Avater ; 

 the faces of the glasses occasioned duplications of the spec- 

 tram,. Which rendered it confused; so that we returned to 

 our English flint glass, which, while perfectly void of 

 colour, combined homogeneity of substance, and accuracy 



of 



