THE PROJECTION MICROSCOPE 203 



and other parts well stained, will probably bear the full cone 

 of light, and be both bright and sharp ; but if we have a slide 

 consisting of only a few of the glands themselves, teased out 

 and mounted separately, the full cone answering to the aper- 

 ture of the lens will confuse detail ; and it must be reduced to 

 a narrower cone in order to get the convolutions of the glands 

 distinctly. Amateur sections are generally far thinner, and 

 better in that respect than can be got from the trade mounters ; 

 but are seldom so well differentiated by staining as the work 

 of the latter. 



It must always be seen, that whatever it is desired to 

 exhibit really is in the slide, satisfactorily, which is not 

 always the case. Let anyone, for instance, try to find a thin 

 section of skin which really exhibits clearly the ' touch- 

 bodies ' as stated on the label, and he will appreciate what I 

 mean ; but it is obvious he must find it, before he can exhibit 

 it. Beyond that, all will depend upon the thinness and the 

 staining ; for it is the want of density in the image that is 

 the chief difficulty in demonstrating such objects. Some 

 further remarks on this subject will be found in 105 in con- 

 nection with the higher power of the electric microscope. 



Botanical sections are beautiful objects, when thin and of 

 proper colour ; if too clear, they occasion the same difficulties, 

 and require us to diminish the cone of light in order to get 

 distinctness. To take an example from this class also : if they 

 could in any way be differentiated by staining, it would be 

 very easy to exhibit the punctated cells in a thin longitudinal 

 section of pine ; as it is, this is a very difficult object with the 

 lime-light, though not at all needing a high power. The cone 

 has to be contracted a great deal, when it can be managed ; 

 with the arc light, so much illumination is left with quite a 

 narrow cone, that it is quite an easy object. 



Foraminifera of all kinds make excellent objects. Poly- 

 cystina project easily by transmitted light, and fairly well on 

 the dark field with spot -lens. Diatoms are unsatisfactory and 



