HINTS TO MICROSCOPISTS. 77 



of candles placed however close to each other. In the absence of wax, 

 composition, and then mould candles, should be sought for ; the great 

 disadvantage, however, of candle illumination, is the unsteadiness of 

 the flame, from draughts and other causes. 



Shades for Argand Lamps. When these are made of metal, they 

 become very hot, and radiate so much heat, as in many instances, when 

 a brilliant illumination is required, and the lamp is placed near to the 

 instrument, to produce considerable inconvenience to the observer. To 

 obviate this, Mr. Nasmyth first used a double metal shade, the internal 

 one being about a quarter of an inch less in diameter than the external ; 

 by this contrivance a current of cold air is constantly passing between 

 the two, and keeps the outer one cool. These shades are best made 

 with a double chimney, so as to encircle the lamp glass and to pass rather 

 above it. This is a great addition to the ordinary shades for lamps. 



If it be desired to illuminate the room generally, as well as the instru- 

 ment, the paper shades answer best, such as are recommended by Mr. 



Gwilt, and made in the following manner : 



/^ ->. " Take half a sheet of good foolscap paper, and 



<^ \ strike thereon two semicircles, as in the diagram, 



r f~\ 4 the longest diameter being thirteen inches, and the 



shorter one four inches, fitting and adapting it 

 to the skeleton sliding frame, as the case may require, and then 

 glueing or pasting the superfluous edges together. When once pro- 

 perly fitted, another pattern may (previously to the glueing of the 

 edges) be traced out and kept at hand, from which any number may 

 at any time be drawn, and a new shade made when wanted, in less 

 than ten minutes. To get rid of the penumbral shadow, lower it 

 down until it becomes almost wholly dissipated. These shades are 

 much less fatiguing to the eye than the contrast occasioned by an 

 opaque body in juxtaposition with a bright light, and they also diffuse 

 a more subdued but useful light around the appartment." (Vol. I. 

 p. 58.) 



Illumination by Reflection. Mr. G. Jackson, to whom microscopists 

 are much indebted for various simple improvements in the construction 

 of the instrument, employs a plano-convex lens of about two inches in 

 diameter, and of four and a half inches focus, silvered on the plane side, 

 and backed with a plate of brass or other metal, to fix a pin in its 

 centre for the purpose of applying it to the microscope. This lens, 

 when so treated, becomes a reflector of about two and a quarter inches 



