434 ON A MONOCHROMATIC LAMP FOR MICROSCOPES, &C. 



same power of giving distinct vision; but his method of observa- 

 tion, which consisted in viewing through a microscope a Nail 

 illuminated in succession with each of the colours of the prism, 

 was by no means calculated to give definite results, and there- 

 fore left the question in all its uncertainty. 



In order to obtain precise indications, which were not ca- 

 pable of being misinterpreted, when applied to practical pur- 

 poses, I formed a spectrum from a luminous disc, by means of 

 a prism of a highly dispersing substance, and with a large re- 

 fracting angle. I then examined this spectrum through a great 

 variety of coloured media, both solid and fluid, and marked 

 the size and shape of the image into which it was converted. 

 The perfection of this image, or its narrowness in the direction 

 of the length of the spectrum, became a precise and unequivo- 

 cal test of the fitness for distinct vision which belonged to the 

 light out of which it was formed. 



By this method of observation, I found that a distinct image 

 of the luminous disc could not be obtained either by produ- 

 cing a Blue or a Green image, and that it was only in the Red 

 portion of the spectrum that such an effect was likely to be ob- 

 tained. In the use of purple glasses, I observed that the 

 middle portion of the Red space was absorbed before the two 

 extreme portions, so that instead of one Red image there were 

 two, quite separate, and tolerably distinct. By increasing, 

 however, the thickness of the plate, the most refrangible red 

 image was absorbed, and the least refrangible one left in a 

 state of the most perfect distinctness. Although I had now 

 determined the part of the spectrum that was best fitted for 

 giving perfect vision, yet the quantity of light extinguished 

 before the insulation of the extreme red ray was affected, was 

 so great as to render the determination of little practical utili- 

 ty, excepting in cases where the outline of an object was to be 

 observed. Had it been possible to insulate the most luminous 

 rays of the spectrum as perfectly as the extreme Red ones, the 



advantage 



