252 PRACTICAL MICROSCOPY. 



invariably have a small cardboard diaphragm, one-eighth 

 diameter, placed beneath them ; the spectrum is then 

 much better defined. With a slide containing a mass of 

 small crystals the object need merely be thrown a little 

 out of focus. 



When observing the spectra of liquids in experiment 

 cells, or through small test-tubes, always slip over the 

 i or 2-inch objective a cap with a hole T V of an inch dia- 

 meter. Slide this cap just sufficiently to bring the small 

 hole a little within the focus of the objective. By this 

 arrangement all extraneous light is prevented from passing 

 up the body of the microscope, except what passes through 

 the object. Unless this precaution be attended to, a false 

 result is sometimes obtained. 



Substances which give bands or lines in the red are 

 best seen by gaslight, while those which give bands in the 

 blue are brought out far better by daylight. Such a 

 specimen as oxalate of chromium and soda is almost 

 opaque by daylight, showing no bands, though when ex- 

 amined by a lamp the spectrum exhibits three beautifully 

 fine lines in the red, two of which are exceedingly delicate. 

 Again, uranic acetate can only be seen to advantage by 

 strong daylight, since the band in the violet would be 

 invisible by lamplight. 



In solids, as crystallised uranic acetate, the tubes should 

 be laid over a piece of black velvet on the stage of the 

 microscope and illuminated with a bull's-eye condenser or 

 side reflector. 



It may be at first thought that absorption in the spec- 

 trum is due to the particular colour only that is to say, 

 a blue would always act as the ammoniacal sulphate of 

 copper ; a brown, as Parisite ; a green as chlorophyll ; and 

 all reds would absorb the same light from the spectrum as 

 a film of blood ; but a few experiments would soon show 



