188 USE OF THE MICEOSCOPE. 



The plane mirror, with some form of condensing apparatus, 

 is required with all powers above that of a hundred linear, 

 when it is wished to examine very delicate objects by arti- 

 ficial light; for this purpose the form of condenser first 

 recommended by Dr. WoUaston for his doublet microscopes, 

 and described in pages 30 and 112, will be found useful; 

 it consists of a tube, in which a plano-convex lens, of three- 

 quarters of an inch focus, is made to slide up and down 

 within certain limits. This tube is fitted to the under part of 

 the stage, and at its lower end is provided with a plate of 

 metal, perforated with a hole of a requisite size, termed a 

 stop ; the rays of light reflected from the mirror, if taken 

 from a white cloud, being parallel, will be thrown upon the 

 lens as such, and by it be brought into a focus upon the 

 object ; this is effected by sliding the lens either up or down. 

 Most objects are best seen when the image of the aperture 

 or stop is also best seen. 



Dr. Goring effected an improvement on Dr. Wollaston's 

 condenser, by putting the stop above the lens, so as to cut off 

 all superfluous rays of light ; but if the original tube be pre- 

 served, the stop above the mirror may still be used, the one 

 immediately above the lens being in no way interfered with 

 by it. Mr. Varley has recommended* a very excellent plan 

 for imitating artificially the light reflected from a white cloud 

 opposite the sun ; this is by covering the surface of the mirror 

 under the stage with the effloresced carbonate of soda or some 

 other white material. On the surface of this the light of the 

 sun is to be concentrated by means of a large condensing lens. 

 The light so reflected is very intense, and forms an excellent 

 substitute for the white cloud, which, as Mr. Eoss states,t 

 when opposite the sun, and of considerable size, is the best 

 daylight, as the pure blue sky, in the same situation, is the 

 worst. 



Mr. Handford has lately described a very excellent substi- 

 tute for the plaster of Paris mirror. He employs a concave 



* Vol. xlix., page 353, of Transactions of the Society of Arts. 

 f Penny Cyclopedia, Art., Microscope. 



