762 LENSES OF VARIOUS KINDS. 



position of the different parts of plants, of the circulation of fluids, 

 and of 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 cautiously ; and the conclusions drawn from it 

 ought to be carefully weighed, more especially when the observations 

 have been made with high magnifying powers. 



Lenses. — 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 (fig. 943, 4), with two convex faces ; plam- 



convex (fig. 943, 3), with one face flat 

 and the other convex; dovMe-eoncave 

 (fig. 943, 2), with two concave faces ; 

 and plano-concave (fig. 943, 1), with 

 one flat and one concave face. Some- 

 times, also, a meniscus (fig. 943, 5) is 

 used, with a concave and a convex face, 

 and a sharp edge, and a concavo-convex (fig. 943, 6), with a concave 

 and convex surface and flat edges. Convex lenses with sharp edges 

 cause parallel rays to converge ; while concave lenses with flat edges 

 cause them to diverge. The lenses used in microscopes are chiefly 

 convex — the concave lenses being employed to make certain modifica- 

 tions in the course of the rays passing through convex lenses, whereby 

 their performance is rendered more exact. The magnifying power of 

 a single lens is inversely as its focal length. The principal focus is 

 the point to which parallel rays converge after refraction. The focal 

 distance of a double convex lens is half that of a plano-convex lens, 

 having the same curvature. 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 to a focus at a certain 

 distance from the lens ; but it is found that no lens with a spherical 

 surface can bring the rays of light to the focus at one point. Hence 

 arises what is called spherical- aberration. In this kind of aberration 

 the rays which pass through the lens near its circumference are 

 brought to a focus nearer to the lens than those which pass through 

 near the centre, hence the objects at the circumference of the field of 

 the microscope are not in focus at the same time as those in the 

 centre. Moreover, the different coloured rays of which white light 

 is composed are unequally refrangible, the violet rays having the 

 greatest and the red rays having the least degree of refrangibUity ; a 



Eig. 943. Different kinds of lenses— 1, Plano-concave. 2, Double-concave. 3, Plano- 

 convex. 4, Double-convex, 5, Meniscus, 6, Concavo-convex, 3, 4, 5, are sharp-t 

 lenses, and cause convergence, 1, 2, 6, are flat-edged, and cause divergence. 



