590 APPENDIX. 



thoroughly in this mucilage. It can then be readily fixed to a small board, on 

 which it must be perfectly dried, whilst a small quantity of the mucilage is occa- 

 sionally poured upon it. Before, however, it has become so dry as to cause the 

 gum to resume its brittleness, delicate sections of the object are made, which are 

 then wetted with a little water on a small glass plate ; the gum imbibes the water, 

 and the object assumes again almost its pristine form. 



Such a preparation with the hand, however, does not suffice for very accurate 

 investigations. Indeed, the view of a section is by no means of importance in 

 many objects; and the thing to be desired is a division of the object into the 

 separate parts of which it is composed : and here we must have recourse to the 

 microscope in order to prepare the object properly. The simple microscope is 

 the most suitable for this purpose, which, especially when Chevalier's or Wol- 

 laston's double lenses are employed, still affords sufficient space between the 

 object and the lens, at a magnifying power of 100 times, to enable us to work with 

 very delicate instruments. The compound microscope has the great disad- 

 vantage, that the object is reversed, requiring, therefore, a very difficult practice 

 of opposing the motion ; and in the next place, our hands are so far removed from 

 the eye as to render their movements uncertain, so that a tearing or crushing of 

 the object at random is scarcely avoidable.* But the greatest hindrances to 

 preparing under the microscope are the instruments. They are, of course, mag- 

 nified as much as the object ; and we soon find that no point is sufficiently fine 

 to enable us to divide the parts of the object with accuracy : very fine needles, 

 which we may sharpen ourselves upon a very fine grindstone, afterwards observ- 

 ing the edge and point under the microscope, are the best for this purpose. 

 English needles, intended for very fine operations, and sharpened in the same way, 

 will answer the purpose. The other difficulty is less easily overcome, viz. the 

 circumstance that our hand is not used to such delicate movements as are neces- 

 sary at a magnifying power of 50 or 60 : practice, however, will serve to remove 

 this impediment. 



After these preliminary considerations, I shall now proceed to the methods by 

 which the object under investigation may be placed in as many different circum- 

 stances as possible, in order thereby to increase the number of points of view. 

 Optical, mechanical, chemical, and physical auxiliaries are here to be distin- 

 guished. They may be called, generally, microscopical reagents. 



a. The Optical. 



First of all, we may remark, that the observer should never limit himself to the 

 observation of an object by one magnifying power alone. It is always advisable 

 to commence with the lower powers^ and gradually to use the higher ones. This 

 mode of proceeding is necessary on account of the fact that the field of vision 

 must diminish in proportion to the degree of magnifying power ; and as it is 

 always requisite, in order to obtain a correct conception, to have a distinct view 

 of the individual parts in all their relations. 



2dly. The changing the direction of the light is also a matter of importance, 

 as we have already explained. 



3dly. It is frequently of advantage to observe an object in a coloured, or, still 

 better, in a monochromatic light : this may be done by using coloured glass for the 

 stage, or employing a spirit-lamp for the illumination, the wick of which has been 

 previously dipped in a solution of common salt, or in which the spirit has been 

 previously diluted with water as much as possible. Both methods, according to 

 Brewster, give a homogeneous yellow light. 



Finally. It is advisable in many cases to observe the object by polarised light, 

 for which purpose a crystalline body, suitably polished for it, is fastened under 

 the table of the microscope. But any working-optician will supply informa- 

 tion on this subject : I need not, therefore, make any further remarks upon it. f 



* An erecting eye-piece in the body of the compound microscope obviates nearly all 

 objections to it as an instrument for dissection. TRANS. 



f Compare Chevalier, Des Microsc. et de leur Usage, pp. 125 128. [For further 

 information on this subject, the English reader is referred to a little work, by Mr. C. 



