xii INTRODUCTION. 



examination were distinctly visible to the unaided eye. The above formal definition of 

 the true use of this valuable instrument is requisite, because it is very frequently used 

 simply as a means of viewing minute objects, and judging of their nature from the mere 

 inspection of them under the conditions in which they naturally or accidentally occur. 

 Such a procedure, the most casual observer must be well aware, is never trusted alone in 

 the examination of objects visible to the naked eye, being almost sure to lead to erroneous 

 conclusions. Consider the common course pursued in the macroscopical examination, or 

 that with the unaided sight, of a body for the first time presented to our notice ! The 

 first point is the examination of its general appearance and colour ; the relative position of 

 the eye of the observer and the object is then changed, so that an idea of its solidity may 

 be obtained ; its weight is next perhaps determined by taking it in the hand ; it is pre- 

 sented to the light in various ways, in order to judge of its transparency, and of the optical 

 properties of its surface. If the object be at a distance, its size is judged of by comparing 

 its apparent size with that of adjacent bodies, whose dimensions are approximately known : 

 and its luminousness is also taken into consideration, it being known generally that the 

 nearer bodies of the same size are to us, the more luminous they appear. The observer 

 then is either satisfied with the conclusions drawn from reasoning upon the results thus 

 obtained, or he makes besides a chemical examination. 



Again, care should be taken to avoid forming an opinion upon the normal or abnormal 

 state of an organic structure, without a previous knowledge of the natural structure of 

 organic tissues. AVe therefore recommend the student, before he thinks of recording his 

 observations, to begin by testing the structure of any objects which may come in his wav, 

 or that of the Test-objects which we have described, according to the rules laid down 

 in the second part of this Introduction. 



It may be remarked for those who have but small means at their command, and who 

 are unable to procure a first-rate English microscope, that perhaps very many of the facts 

 elicited by the use of this instrument have been determined by our continental neighbours 

 with far less perfect instruments, who have made up for the imperfections of their instru- 

 ments by extreme patience, care, and repeated observation; which can be done to an 

 extent that would scarcely have been anticipated. 



We have alluded to these sources of error merely for the purpose of warning future 

 observers, and impressing upon them the importance of making themselves acquainted 

 with the difficulties attendant upon microscopic investigations, and with the best means of 

 overcoming them. In fact, it may be briefly stated that the object of the present work is 

 to guide the microscopist in his researches, to give him a notion of the manner of making 

 these researches, also some account of the characters, microscopic structure, and properties 

 of objects in general, and to show how he may most easily arrive at satisfactory results. 



But there are difficulties inherentky connected with the examination of microscopic 

 objects, which are not encountered when objects are examined with the naked eve. One 

 of these is that, with the ordinary microscope objects are only viewed with one eve ; hence 

 we lose the direct power of distinguishing solidity, &c, and are compelled to resort to 

 indirect means for these purposes. This difficulty is to some extent overcome by the con- 

 struction of binocular microscopes. Again, the ordinary objects around us are also usually 

 viewed by reflected light, whilst with the microscope they are mostly viewed by trans- 

 mitted light, and we are consequently much less practised in judging from the appearances 

 of objects thus illuminated, and are therefore liable to err. 



