X INTRODUCTION. 



normal or abnormal state of an organic structure, without a previous knowledge of the 

 natural structure of organic tissues. And so imperfect is the system according to which 

 investigations are conducted, that we not unfrequently hear of a morbid structure having 

 been submitted to two or more observers, each of whom gives a different report, even as to 

 its morphological components. 



Through such and other similar circumstances, the microscope has given rise to most 

 absurd errors, which might have been easily prevented by a slight knowledge of its proper 

 use ; thus, to take a few examples : the fruit of the mulberry has been mistaken for entozoa ; 

 calcareous corpuscles have been regarded by several observers as ova, and the appearance 

 arising from the presence of concentric laminae has been interpreted to be the coils of an 

 enclosed embryo ; similar corpuscles have also been regarded as nucleated cells, and again 

 as blood-corpuscles : minute fossils in chalk have been strung together with portions of 

 vegetable tissue and (perhaps) the sjiores of algae, to constitute diiferent stages of a fungus : 

 minute hairs projecting from the surface of a membrane have been declared to be spicula 

 within subjacent cells ; and quite recently, one writer states that certain minute bodies w^iich 

 he has examined are either blood-corpuscles or the spores of fungi, but which, is doubtful ! 

 while another recounts, how by fortunate accident he discovered that corpuscles, which he 

 had regarded at first as consisting of fat, were afterwards found accidentally to consist of 

 calcareous salts ! Again, we read in physiological works of the yolk-cells and the coloured 

 oil-globules of the yolk, and a beautiful function of assimilation has been attributed to 

 them : but they exist only in the imagination of the authors, who have regarded the one as 

 cells simply because they are round, and the other as consisting of fat, because they are 

 highly refractive. Since the publication of Schleiden's cell-doctrine, almost everything 

 round has been regarded as a cell ; any single body within this, or w4iere there are several, 

 the largest, has been regarded as a nucleus, and any spot within the nucleus has been viewed 

 as a nucleolus. Whereas many of the so-called cells are homogeneous s])heres ; many of the 

 nuclei are vacuoles, and a true nucleolus is very rarely found except in the books. 



Hence, the very utility of the microscope as a means of investigation has been called in 

 question. But it must be borne in mind, that all the known errors committed in the 

 application of this instrument have been corrected also by its use ; so that the fault cannot 

 be attributed to the instrument, but to the observer. 



As error is almost invariably associated, at least temporarily, with improvement or advance 

 in knowledge, it might be urged that the above errors w^ere unavoidable. A little reflection 

 and the perusal of the means given presently, will show, however, that they have arisen 

 from extreme carelessness, and from ignorance of the means of distinguishing appearances 

 from realities. And 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, the possession of 

 which is commonly regarded as all that is requisite, that perhaps nine-tenths of the facts 

 ehcited by the use of this instrument have been determined by foreigners with far less 

 perfect instruments, and that a large part of the results obtained in this country with the 

 best microscopes, are either erroneous or have been published as original after having been 

 previously made known in foreign works. 



We have alluded to these errors 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 micro- 

 scopist in his researches, to give him a notion of the manner of making these researches, 

 also some account of the microscopic structure and properties of objects in general, and to 

 show how he may most easily arrive at satisfactory results. A knowledge of the structure 



