THE PROJECTION MICROSCOPE 205 



still thinner troughs are required, a thin ivory card, or piece 

 of thick cartridge-paper, will give the required thickness. 

 Botterill troughs, in which a rubber ring is pinched by screws 

 between two glasses, are handy for greater thicknesses, and 

 can be cleaned at pleasure. 



The movements of living diatoms are easily shown, in- 

 cluding Bacillaria paradoxa ; these only need a drop of the 

 water on a plain glass, with a slip of cover-glass held on by 

 capillarity. Volvox is easily shown 8 or 10 inches diameter, 

 requiring a trough with thin glass face, and enough fluid 

 between the glasses to allow it to roll freely, else there will be 

 no movement. Greater magnification is only hindered by the 

 thickness of the globe baffling any focussing of its image with 

 higher powers. Of infusoria, some such as stentors are easy 

 objects ; Vorticella is rather difficult. The chief difficulty is 

 that their thickness and their motion make it hard to bring 

 high power to bear upon many of them. 



Hydra, water-fleas, cy clops, and the various larvae of the 

 gnat family, make good objects. The larva of the true gnat is 

 very opaque, and too thick at the thorax to focus very sharply ; 

 but some of the ' blood-worms ' make capital objects. The 

 corethra (' phantom ') larva is an excellent object, owing to its 

 transparency, and so is that of the May-fly. The small worm 

 found in clusters in the mud of a pond, called Tubifex, is a 

 truly imposing object on the screen, appearing both in shape, 

 size, and even colour, very much like a gigantic python. The 

 microscopist will however readily multiply the list for himself, 

 and will find the manipulation precisely the same that he is 

 accustomed to. 



For displaying the circulation of the blood in the foot of a 

 frog, a frog-plate may be used ; but personally I prefer the 

 homely and old-fashioned plan of employing a sole of thin 

 cork, with a hole cut in it just large enough to show 

 the necessary portion of the web. By tying a short, looped 

 bit of thread round each toe, the foot can be held in position 



