214 Qn the Decomposition of Light. 



cppt at the place left for tn- transaiission of the ravs; this 

 prism being continutd so as to be moveable in different 

 directions, as occasion might lequire: 6, a board covered 

 ■with white cloth, at the distance ot 422 centimetres (13 

 feer, S inches) from the Ien». All these were placed, kept, 

 or brought into the proper directions, suited to their several 

 pui-poses, and to the course of the sun. 1 he place too was 

 so contrived, as to be rendered pretiy dark at pleasure. 

 Having taken every possible care in arranging our appa* 

 ratus, we were able to obtain every day, when it was fine 

 weather, a \-c\-y simple spcctruin for several hours ; which 

 was quite sufficient tor our various experimenlsJ, at some of 

 which Messr*. Berlhollet, the father and son, Mr. Laplace, 

 and other gentlemen were present. 



The spectnuii was very distinctly bounded by two rec-» 

 tilinear, and perfectlv parallel sides. Its length was a little 

 more than 24 centimetres (9. 36 inches) Its breadth was 

 ■^ o^ its length, when the aperture in the diaphragm was 

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

 half, and the contraction of the spectrum was proportional, 

 the breadth then not being more than -J5-; and lastly, by 

 diminishing the aperture, it was reduced to -j-'-j- of the length. 



As to the strength of the colours, they were vivid and 

 bright as might be expected. The impression on the eye 

 was such, on account of the narrowness uf the image, that 

 at a few metres (two thirds of a foot) from the cloth the 

 spectrum 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 

 pf the image depends on the conformation of the eve, and 

 is connected with some other phaenomena, of which I may 

 hereafter give an account. 



The distinction of the colours, and their separation into 

 seven classes, was likewise one of the objects of our in- 

 quiry. Though the existence of this distinction was per- 

 ceived, it must be confessed, that it was not easy to trace 

 all their divisiotis. I made gome attempts to effect it, the 



narrative 



