390 



Z'^'^LOGY. 



[Sect. XI. 



completely ; in this diaphragm there may he a small 

 orifice with a slide, to let in a pencil of light for small 

 objects. The whole microscope should he screwed into a 

 solid block of oak, and not into the lid of the box as is 



usual. 



The mirror should be capable of movement in every 



direction, and of sliding up and down the column ; on 



one side there must be a large concave mirror, and on 



the 



mirrors 



fitted water tight m caps, made to screw off and on : and 



two o 



:4Nl 



on a h~ng voyage, as salt water spilt on the mirror easl!: 

 deadens the quicksilver. A small cap is very convenient 

 to cover the mirror when not in use, and often saves it 

 from being wet. The vertical shaft by which the lenses 



are moved up and down 



T (as ' 

 form) 



^^^ 



^ s 



tX 



djiist- 



there should be on both sides large milled he 

 such, there is no occasion for fine moYemeiits 

 ment, which alwavs tend to weaken the instrument. The 

 horizontal shaft should be capable of revolving, and 



(f 



right and left hands) , but the left milled head must be 



quite small, to allow of the cheek and eye approacliir;^ 

 close to the lenses of high power. The horizontal shaft 



must come down to the stage. 



The most useful lenses are doublets of 1 inch and 

 6-lOths of an inch (measiu*ed from the lower glass of the 



doublet) in focal distance ; a simple lens of 4 or 5-lOths 

 of an inch is a very valuable power ; and, lastly, C:d- 

 rington lenses (of the kind sold by Adie of Edinburgh), 





