ON INSTRUMENTS AND THEIR USE. 11 



within about two inches from its surface. The 

 reason of this is, that the crystalline lens of the 

 eye, in assuming a more convex shape through the 

 relaxation of the ciliary muscle, becomes overtaxed 

 at this distance. If, now, a sufficiently convex lens 

 be placed between the object and the eye, so as to 

 enable the divergent rays to be accurately focussed 

 upon the retina, the difficulty will be overcome, 

 and, theoretically, microscopical vision would be 

 illimitable. But, is it so ? Most certainly not. 

 The employment of artificial substances, such as 

 crown and flint glass, diamonds, &c., although they 

 considerably extend the power of sight, do not do 

 so ad infinitum. Here the limitation is purely 

 material, as distinguished from the former instance, 

 the human eye, which is defective not only materi- 

 ally, but physiologically. 



The worker in the field of microscopical research 

 need not, however, be appalled by these statements, 

 for it will be found that the human organ of vision, 

 in conjunction with the excellent appliances of 

 modern invention, will enable him to approach, 

 and sometimes even to solve satisfactorily, many of 

 those philosophical problems which underlie the 

 evolution of things, both animate and inanimate. 

 In exemplification of this, a few instances may here 

 be recorded. 



The practical geologist who sallies forth into the 

 field with lens in hand, may gather during his walk 

 a variety of rocks, which, from their cosmological 

 structure, point to an igneous origin ; some of the 

 specimens are coarse grained, whilst others defy the 

 utmost scrutiny of the eye. The microscope is 

 brought to bear upon the question, and he finds 



