INTKODUOTION. xiii 



Another, but a lets important difficulty in microscopic investigations, or at least mani- 

 pulations, consists in the image of the objects being inverted. Erecting eyepieces, as they 

 are called, will obviate this difficulty ; but as they are expensive, and interfere with the 

 distinctness of the images of the objects, and as the difficulty is to a great extent got over 

 by practice, the\' are rarely used. 



Another very serious source of error lies in the tendency to reason from analogy as to 

 the structure or nature of a body viewed under the microscope. Any one who pursues 

 this course has his mind prejudiced by preconceived notions, and becomes in fact no 

 observer at all. 



It need, moreover, be merely remarked that the ordinary appearance of objects to the 

 naked eye depends in all cases upon a molecular structure, which is generally microscopic, 

 the ordinary appearance being the optical result or expression of this structure ; and since 

 totally dissimilar microscopic structures may present similar appearances to the unaided 

 eye, j udgment as to the nature of the former founded upon the latter can be of but little 

 value. The reader will remember that the common capability of distinguishing objects or 

 structures by their appearance has been derived, so to speak, from practice and experience 

 of effects ; and when we bear in mind that the experience and practice in the study of the 

 causes are attainable, the superiority of the latter must be evident. 



Next to the improvement effected in the optical construction of the microscope during 

 the last few years, must be placed that of the method of investigation. Formerly almost 

 all microscopic bodies possessing different forms and appearances were considered distinct 

 beings, and were named accordingly. By the present method, prolonged observation is 

 adopted to follow the changes which the individual bodies undergo ; whence it has resulted 

 that numbers of them have been found to be simply different stages of each other. Thus 

 a large amount of useless nomenclature and confusion is being removed from the domain 

 of the microscopic world. 



Above all, however, it must never be forgotten that microscopic investigations require 

 more time and patience than perhaps any others, even in regard to the determination of 

 simple points of structure and qualitative composition. In fact, notwithstanding the innu- 

 merable observations made upon the more minute objects, such as the scales of insects, the 

 markings on the valves of the Diatomaceae, the fibrillse of muscular fibre, etc., such dif- 

 ferences of opinion are still entertained, that it can by no means be asserted that the 

 structure of these bodies is positively known. 



The time has passed at which the value of microscopic research could be called in 

 question. The wonderful insight gained b} r its use into the structure and functions of the 

 various organic beings belonging to the Animal and Vegetable Kingdoms, the aid it has 

 afforded Geolog}*, the so-called practical applications it has permitted in improving the 

 arts, in detecting adulterations, and in defeating crime, moreover the almost positive 

 certainty we have obtained that it is capable of displaying all the real structure which 

 bodies possess, save that of their ultimate mokcularity, which will probably always bo 

 hidden from us, are sufficient to deprive this question of any interest. 



Lastly, if it were required to prove design in the creation, this could not be more easily 

 effected than by the examination of the structure of microscopic organisms. 



We have expressed our intention of not entering upon a description of the microscope 

 as an optical instrument, because it would have been requisite to tread widely the field of 

 general optics, which our space does not permit. We would therefore advise those who 



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