xxviii INTRODUCTION. 



extensive analogical reasoning ; a few sections and a glance, — all the parts which appear 

 round are cells, the elongated parts, fibres, or tubes, &c., — a hasty comparison of these appear- 

 ances with others observed in other organisms, with a little analogical reasoning, will yield a 

 clear and lucid physiological explanation, and perhaps develope a beautiful homology. The 

 whole may be done in a few minutes, or at the most a few hours ! 



It is very different, however, with the determination of the histological or true structure. 

 Frequently a week or a month must be devoted to the determination of a single point. 

 Take the instance of a hard structure — a piece of the skeleton of one of the Invertebrata. 

 A few sections made by an artisan will exhibit cells, laminae or fibres, according to the pre- 

 conceived notions of the observer ; whilst the histologist will not express an opinion until 

 the inorganic matters have been removed by long maceration in acid, the calcareous salts 

 thoroughly washed away, and attempts been made to resolve the organic basis into its 

 histological elements by appropriate means. This may be the work of months, and may 

 require very many experiments to be made, and no mean knowledge of particular branches 

 of science for guidance in the selection of appropriate agents requisite for their performance. 

 Yet the two kinds of observations make equal show upon paper. We shall have frequent 

 occasion to use the above words in the restricted sense ; hence this should not be forgotten. 

 The word analysis will have the same meaning as that generally attributed to it, the ultimate 

 products being morphological. 



A general method for determining the structure of objects can hardl}'^ be laid down; it 

 must varj^ so greatly according to the nature of the objects and their size. The first point 

 is to render them transparent, if not already so. This may frequently be done by immersion 

 or maceration, if dry, in water or oil of turpentine ; sometimes the aid of heat is necessary, 

 and they may even require to be boiled in these liquids, either upon a slide placed upon the 

 tin plate over the flame of a spirit-lamp, or in a small tube. Sometimes sections require to 

 be made, and these treated in the same manner. If soft, their elements may be separated 

 by the aid of needles ; sometimes pressure will answer the same purpose. 



When the object is very minute, it will frequently be desirable to examine both sides of 

 it with high powers. Hence it must not be placed upon an ordinary slide, on account of 

 the thickness of the latter, but must be supported upon, and covered by thin glass. The 

 best plan is to keep a number of slides, made of tolerably stout card or card-board, each 

 having a piece cut out of the middle. A piece of thin glass, rather larger than the 

 aperture, should then be cemented by marine glue or Canada balsam to one side of the 

 card, and a slide is thus prepared ; the thin glass cover is then applied as usual. 



A great advantage of this method of temporarily or permanently mounting objects is, that 

 the card-board being flexible, there is no fear of injuring the object-glass, even if it should 

 come into contact with the glass cover. If the object be very small and its structure 

 very delicate, it must be crushed, so that some of the fragments may lie perfectly flat upon 

 the slide. See also the article Preparation. 



But the points to be determined in regard to the diff'erent parts of an object, may be best 

 treated separate^. 



The examination of a microscopic object must comprise, — «, the microscopic analysis, 

 including,—!, the form ; 2, the colour; 3, the structure of the surface; and 4, the internal 

 structure : b, the histological analysis, in the sense already explained : c, the qualitative 

 chemical composition; and d, the measurement. 



A. Microscopic Analysis. 



1. The Form.— a. This is usually judged of from the outline, as seen by transmitted 

 light, and often erroneously. Where a low power is used, the upper surface of an object 



