APPENDIX. 



I- ON THE USE OF THE MICROSCOPE IN BOTANICAL RESEARCHES. 



1191. THE Microscope (fux,po;, small, and oxoxia, 1 see) is an instrument 

 fur enabling the eye to see distinctly objects which are placed at a very short 

 distance from it, or to see minute objects that would otherwise be invisible. 

 It has been used with great success in the examination of vegetable structure. 

 To it we are indebted for a knowledge of the various vessels and cells which 

 enter into the composition of the different parts of plants, the circulation of fluids 

 and ciliary movements, as well as for the facts connected with the development 

 of the embryo. It is an instrument, however, which requires to be used cau- 

 tiously ; and the conclusions drawn from it ought to be carefully weighed, more 

 especially when the observations have been made with high magnifying powers. 



1192. incuses. Before proceeding to notice the construction of simple and 

 compound microscopes, it will be advantageous to notice the different kinds of 

 lenses used, and the sources of error which require to be guarded against in 

 their preparation. The chief forms of lenses used are, the double-convex, with 

 two convex faces ; plano-convex, with one face flat and the other concave ; 

 doubk-concave, with two concave faces ; and plano-concave, with one flat and 

 one concave face. Sometimes, also, a meniscus is used, with a concave and a 

 convex face, and a sharp edge. In the use of ordinary lenses, there are sources 

 of error from the form of the lens, and the nature of the material of which it is 

 made. When parallel rays fall on a double-convex or a plano-convex lens, they 

 are brought into a focus at a certain distance : but it is found that no lens with 

 a spherical surface can bring the rays of light coming from one point exactly 

 into the focus at another point. Hence arises what is called spherical aberra- 

 tion. Moreover, the material of which the lens is made acts differently on the 

 different portions of each ray, and separates the white light into different colours, 

 which have various degrees of refrangibility. This gives rise to chromatic 

 (xguftu, colour,) aberration. To remedy these defects, certain combinations of 

 glasses have been adopted, so that the light traversing one lens through the 

 centre may pass through near the margin of another. The confusion produced 

 by these aberrations may be greatly diminished by diminishing the pencil of 

 light ; for instance, by employing a stop or diaphragm, which lessens the aper- 



