MICROSCOPIC OBJECTS. 41 



mounted in Canada balsam, has tended greatly to assist 

 us in discovering the real form of these members. The 

 number of suction-pads to each foot is not constant ; 

 some flies have two, others have three, and in others they 

 are altogether absent. The anterior and middle pairs of 

 feet of the male Dytiscus, as before remarked, are fur- 

 nished with curious disc- or cup-shaped appendages on 

 the inside of the leg. — Magnii^dng power 30 to 100 

 diameters. 



Fossils^ — In several argillaceous strata and limestones 

 minute shells may be readily discovered. The specimens 

 should be cut into thin sections. Among the many im- 

 portant facts brought to light by the study of the con- 

 tents of flints, limestones, &.c. is the fact of their 

 comparatively recent formation, it being clear that the 

 animals, the remains of which they inclose, must have 

 existed prior to the formation of those stones. Thin 

 sections of the Brighton pebble are distinguished by 

 inclosing curious egg-shaped bodies ; these are figured in 

 Pritchard's ' History of Infusoria,' plate 12. The flints 

 from the chalk deposits are mostly sihcified sponges, and 

 usually contain minute organic remains similar to those 

 found in recent sponges of the present time. — The mag- 

 nifying powers for these examinations vary greatly, ac- 

 cordmg to the nature of the fossil ; average, 60 to 450 

 diameters. 



Hair of Animals, &;c, — By ordinar}^ vision most hairs 

 appear cylindrical; but by the assistance of the micro- 

 scope great variety in their forms may be detected. When 



