XXX. — Description of a Monochromatic Lamp for Microsco- 

 pical purposes, fyc. with Remarks on the Absorption of the 

 Prismatic Rays by coloured Media. By David Brewster, 

 LL. D. F. R. S. Lond. & Sec. R. S. Edin. 



(Read April 15. 1822. ) 



JlN a Paper on Vision through coloured Glasses, which I had 

 lately the honour of submitting to the Society *, I pointed out 

 the advantages of coloured media in Microscopical and Teles- 

 copical observations. Having experienced the great utility of 

 Green and Red lenses, in developing vegetable structures that 

 required to be examined with high powers, I was anxious to 

 derive from this new principle all the advantages which it ap- 

 peared to possess. In attempting to do this, it became necessary 

 to ascertain the power of giving distinct vision, which belonged 

 to each separate colour of the spectrum, and though I had 

 stated in my former paper, " that it was difficult to discover 

 " any reason why one coloured medium should be preferred 

 " to another, provided each of them transmits equal quantities 

 " of homogeneous light ;" yet it was desirable to put this 

 theoretical opinion to the test of direct experiment. Sir Wil- 

 liam Herschel j" had long ago investigated this point in refe- 

 rence to the use of coloured media for solar observations, and 

 had concluded that every colour of the spectrum possessed the 



3 i 2 same 



* See the Edinburgh Philosophical Journal, vol. vi. p. 102. 

 •j- Philosophical Transactions, 1800. 



