12 METHODS OF MICROSCOPICAL RESEARCH. 



that a power of 500 diameters is generally the 

 utmost degree of amplification he will require to 

 employ ; but that for all practical purposes powers 

 of from 20 to 100 diameters suffice. "With the 

 assistance of the microscope he is enabled to pro- 

 nounce with decision that the rocks are igneous ; 

 and more, from analytical and synthetical experi- 

 ments he can show that certam coarse varieties, 

 which are thoroughly crystalline (the crystals being 

 simply held together by adhesive and cohesive 

 forces, without the necessity of an interstitial bind- 

 ing substance), are of deep-seated origin, and con- 

 solidated under conditions of enormous pressure 

 and length of time. He is, in like manner, able to 

 affirm of the other varieties, what their mineral 

 constituents are, or have been, and how they came 

 to assume their present states. Thus he builds a 

 part of the fabric of geological philosophy, and with 

 what ? with a comparatively low power of the 

 microscope. 



To take an example from the organic world ; the 

 questions of the function of various parts of the 

 body, are very often arrived at through a minute 

 study of its members. The form and general ap- 

 pearance of the cells of glands such as the salivary 

 glands, point to the functions they perform, whether 

 they secrete or absorb, and how and when they 

 perform their duties. The study of amoeboid, and 

 even of ciliary motion, under the microscope, does 

 not require a power of magnification much beyond 

 700 diameters, whilst the life history of the minute 

 forms of life known as germs (bacteria, fyc.), may 

 be readily comprehended by the use of from 700- 

 1,200 diameters. It is only when such things as the 



