SIMPLE AND COMPOUND MICEOSCOPES. 763 



leng therefore breaks up a ray of white light into its constituent 

 colours, so that a colourless object appears coloured. This is termed 

 chromatic (^gSi/^cc, 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 

 lessened by diminishing the pencil of light ; for instance, by employing 

 a stop or diaphragm, which lessens the aperture of the lens and cuts 

 off the peripheral rays. In lenses of low power, such as are used in 

 the simple dissecting microscope, these aberrations do not cause much 

 confusion. It is only when high powers are required that these 

 ab&rations must be done away with. The invention of WoUaston's 

 doublet with two lenses, and Holland's triplet with three, was with the 

 view of diminishing, as far as possible, these aberrations. They were 

 aplanatic (a privative, •s-Xai/aw, I wander), i.e. they remedied spherical 

 aberration, but coloured images were stUl produced. Their lenses were 

 constructed of the same kind of material ; and it was found that in 

 order that lenses might present the object uncoloured, j 

 or be what is called achromatic (a, privative, and ;^jS^a, 

 colour), it was necessary to use two glasses of different 

 refractive power. Achromatic lenses, or such as are nearly 

 free from chromatic aberration, are constructed by placing 

 together glasses of different dispersive powers, and of 

 different forms. The usual achromatic lens consists of 

 a double-convex lens, made of plate or crown glass, and 

 a plano-concave, made of flint-glass (fig. 944), fitted '^' ' 

 accurately to it, and cemented by Canada balsam. 



MiOEOSCOPES are of two kinds-^Simple and compound. By the 

 Simple Microscope objects are viewed through a single lens, or through 

 two or three lenses placed together, so as to form doublets or triplets. 

 The glass is arranged so that it can be brought over the object, and 

 adjusted, by means of a rack and pinion, or by some other contrivance, 

 to its exact focal distance — the object, when opaque, being seen by 

 light thrown from above, and when transparent, by light transmitted 

 from below. This instrument, when used with single lenses or 

 doublets, is the best for ordinary botanical investigations, more especi- 

 ally for dissections. The combination of three lenses approaches too 

 near the object to be easily used. A very high power may be obtained 

 by doublets formed of plano-convex glasses, or by means of the lenses 

 termed Coddington's or periscopie, consisting of two hemispherical 

 lenses, ■ cemented together by their plane faces, having a stop between 

 them, or rather having a groove in the whole sphere filled with 

 opaque matter. The chief objections to the simple microscope are 



Fig. 944. u. An achromatic and aplanatic lens, consisting of a double-convex lens of 

 plate-glass, and a plano-concave of flint-glass, b, Section of the plano-concave lens. 



