1S6 PPIILOSOPHY OF ZOOLOGY. 



the methods ah-eady pointed out, another instrument is em- 

 ployed, the Microscope — scarcely more remarkable for the 

 extent of discovery which it has yielded, than for its 

 fruitfulness of error. In using this instrument, so much 

 depends on the dexterity of the observer, the condition of 

 the eye, and the distribution of the light, that it is exceed- 

 ingly difficult to avoid error. We have already pointed 

 out, in the case of the shape of the globules of the blood, a 

 very remarkable example of discordant results, in the micro- 

 scopical examination of the same object by different observers. 

 Numerous examples of the same kind might be referred to, 

 in the highly magnified representations of the parts of the 

 same animals, as given by different naturalists. Indeed, 

 no great confidence can be reposed in the accuracy of such 

 designs, where the sources of deception are so numerous, 

 and where the imagination is often pei-mitted to guide the 

 hand, while delineating appearances but obscurely per- 

 ceived by the eye. 



In the examination of objects with the microscope, it is 

 of great importance to employ, at the first, a small magni- 

 fying power, to enable the eye to take in at one ^'iew a 

 considerable extent of surface, and perceive the relative po- 

 sition of the different parts. Higher magnifiers may then 

 be applied in succession. Single lenses are seldom used, 

 where the focus is less than one-fifth part of an inch. At 

 this focal distance, it becomes exceedingly difficult to avoid 

 touching the objects, and if their surface is unequal, the 

 relation of the small portion which is distinctly seen, can- 

 not be traced to the surrounding parts, the image of which 

 is obscure. It is of importance, therefore, to endeavour to 

 gain a distinct, rather than a highly magnified image of an 

 object. 



The compound microscope is chiefly employed where 

 amusement is the object in view. In using it, the eye is 



