MICROSCOPIC OBJECTS. 63 



then mounted in Canada balsam between plates of glass, 

 it will prove an admirable object for the gas-microscope. 

 In this example the cells containing the colouring- matter 

 are large, and therefore suited for low powers. 



Sole-skin. — This is an interesting opaque object. It 

 should be cut or punched into pieces about half an inch 

 in diameter, which should be attached to a black disc or 

 on a wood sUde. The mode of attachment of the scales 

 to the skin and the serrated form of the former deserve 

 notice. — Magnifying power 30 to 60 diameters. 



Spiders. — This order of annulose animals has been 

 little attended to by microscopists, owing to the difficulty 

 of preserving the specimens entire. The most remarka- 

 ble point is the grouping of the eyes, which, unlike those 

 of insects, are single. In one genus (NopsJ, the species 

 have only two eyes ; but spiders are mostly provided with 

 six or eight. In some large foreign specimens the hairs 

 are curiously marked. The respiratory apertures are 

 usually four, placed near the spinnerets, towards the base 

 of the abdomen. 



Spiders' Web (Cluhionaatrox). — ^The web of this spider 

 which Mr. R. Potter considers a very difficult test 

 object, has in recent specimens "the complete structure of 

 a regularly- woven net" upon it. It requires a magnify- 

 ing power of 400 to 500 diameters to penetrate its struc- 

 ture. "The Clubiona atrox is found in the crevices of old 

 walls, and may be known by its making an irregular fleecy- 

 looking web, a thread of which when examined under the 

 microscope " (by stretching a short length across a bent 



