THE MICROSCOPE AND ITS ACCESSORIES. 



rod, which, bent in a gentle curve, conducts the light from the 

 flame to the under surface of the stage, the mirror in this case not 

 being used. 



Perhaps the most useful lamp is that known as the albo-carbon 

 light. It is the one I am in the habit of using when especially 

 in winter good daylight is not available. Ordinary gas is used, 

 but the gas has to traverse a chamber containing naphthaline before 

 it reaches the burner. A very white light is thus obtained, and 

 can be used without the intervention of blue glass or a blue copper 

 solution. 



If expense be no bar, then a small incandescent electric light is 

 most useful. 



19. Dissecting Microscope (figs. 24, 25). This is very useful. 

 The lenses usually employed magnify from 5 to 20 times linear, 

 and are fitted into a framework which can be raised or depressed, 

 so as to bring the object distinctly into focus. 



20. Method of Measuring the Thickness of Cover-Glasses. 

 Thick cover-glasses are of no use, and thin ones extra thin, so 

 called are to be pre 



ferred. A convenient 

 thickness is o. 16 mm. 

 For high powers this 1 

 is essential, and it is 

 well to measure the 

 thickness of the cover- 

 glasses, and to reject 

 all those above a cer- 

 tain thickness. This 

 is conveniently and 

 rapidly done by means 



of the instrument FlG . ^. Cover-glass Tester. 



shown in fig. 26. A 



clip projecting from the box fixes the cover-glasses, and the thick- 

 ness is given by an indicator moving over a divided circle on the lid 

 of the box. The divisions show hundredths of a millimetre. 



21. Camera Obscura Shade. When one has to continue observing 

 a microscopic object for a long time, it is convenient to shade the 

 eyes from all light reaching them, except that transmitted through 

 the eye-piece. This is best done by a blackened screen cither fixed 

 to the microscope or arranged in front of the microscope. 1 Some 

 observers place the microscope in a dark chamber, allowing light to 

 fall upon the mirror through an aperture in its front wall Flbgcl 2 

 has designed a camera obscura for this purpose. 



1 Schieffcrdecker, Archivf. iviss. Mikr., p. 180, 1892. 



2 Zoolog. Anzeiger, by V. Cams, p. 566, 1883. 



