72 Vegetable Phyftiohffy. 



hrouo^lit to a focus on the sensitive surface at the back of the 

 eye, and therefore vision is indistinct. When a small lens or 

 magnifvina: glass is interposed in front of the eye, it condenses 

 the rays of light (just as it condenses the rays of the sun when 

 used as a burning-glass), and brings them to a focus on the 

 retina, so that the object is seen clearly. At the same time, 

 however, from the mode in which the rays of light are bent by 

 the lens, the object is magnified ; it appears larger and at a 

 greater distance from the eye than it really is. All this may be 

 proved with any common magnifying-glass, and is indeed no 

 more than the ordinary action of the spectacles used by old per- 

 sons for reading and other purposes, when they suffer from dim- 

 ness of sight of near objects. The principles involved here are 

 as certain as any that have been ascertained in any department 

 of science ; and the construction of the more powerful magni- 

 fying instruments used by microscopic observers is regulated 

 by* the same laws, only in a more complicated application. 



In most physiological observations with the microscope, there- 

 fore, where the instrument is good, little doubt need attach to- 

 those results which depend upon things which are clearly seen 

 by an experienced observer. But when it is considered how 

 delicate and minute are the objects investigated, compared with 

 those which we ordinarily see, and when it is remembered that 

 in many cases we are reduced to seewg alone, and cannot touch, 

 taste, smell, weigh, or otherwise examine, so as to control the 

 perceptions of the eye, it is not strange that in the earlier stages 

 of the application of the microscope much misapprehension and 

 many errors should have arisen. To work properly with the 

 microscope the eye requires a special education, affording it an 

 experience which compensates for the absence of those checks 

 which in the case of ordinary vision are supplied by the other 

 senses. Perhaps the greatest of the practical difficulties, how- 

 ever, of microscopic investigation in the present da^^, lie in the 

 dissection and preparation of the objects to be observed, which, 

 from their minute dimensions and often excessive delicacy, call 

 for the exercise of a manipulative skill demanding long practice 

 for its acquisition, and very considerable perseverance in its 

 application. This is especially the case in all those Inquiries 

 on which the most important physiological questions hinge. 

 But there is no greater obstacle here tlian is met with in the 

 pursuit of the other branches of natural science, and indeed in 

 any thorough application of human intelligence. Nothing solid 

 is to be learned or gained by man by the use of his faculties in 

 any field without serving an apprenticeship. Chemistry worJied 

 long through doubt and obscurity, and has now proved itself one 



