1813.] Philosophical Transactions for 1813, Part I. 233 



Dispersive Powers. 



Chromate of lead, greatest 0*400 



Ditto, least refraction 0*262 



Realgar 0*255 



Oil of cassia 0139 



Sulphur 0*130 



Phosphorus ; 0*128 



Flint glass (highest) 0*052 



Diamond V 0*038 



Water 0035 



Fluor spar 0022 



Cryolite 0-022 



4. On the Existence of two dispersive Powers in all doubly 

 refracting Crystals. When substances refract doubly, one image 

 is always more coloured than the other. Hence it is obvious that 

 such substances possess a doubly dispersive power. The follow- 

 ing table exhibits the double dispersions of various bodies: — 



Chromate of lead, greatest 0*400 



Ditto, least 0-262 



Carbonate of lead, greatest - 091 



Ditto, least 0066 



Carbonate of strontian, greatest . . . .0-016 



Ditto, least 0*027 



Calcareous spar, greatest 0-040 



Ditto, least 0*026 



X IV. An Appendix to Mr. Hare's Paper on Vision. By Sir 

 Charks Blagden, F.R. S.] Sir Charles states his concurrence 

 in opinion with Mr. Ware, that near-sightedness comes on at 

 :ni early age, and that it is almost confined to the higher ranks. 

 He concaves it to be owing to the habit acquired by such 

 young people of confining their attention to near objects. He 



ribes his own case as an illustration. At the age of four, 

 when be learned to read, he was not near-sighted ; but being 

 much addicted to reading, he became slightly so at eight. He 

 was dissuaded from using a glass, and struggled on with a cer- 

 tain degree of near-sightedness till the age of thirty. He then 

 Died the glass called No. 2, which in a few years he laid aside 

 tor No. 3, then for No. 5, which he still employs. 



XV. A Met bid of drawing extremely fine Wires. Yiy Wil-. 

 Uam Hyde Wollaston, M.l). Sec. U.S.] The method employed 

 by this ingenious philosopher was to take a small platinum wire, 

 put it into the centre of a mould, and fill the mould with fine 

 silver. The silver is then to be drawn into a wire as fine as 

 thought requisite. The silver wire thus obtained is bent into the 

 form of an U, making two hooks at its upper extremities; and 



