612 SIMPLE MICROSCOPE COMPOUND MICROSCOPE. 



ture 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 will not cause 

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

 must be done away with, the aperture being increased without interfering with 

 definition. The invention of Wollaston's doublet with two lenses, and Hoi- 

 Land's triplet with three, was with the view of diminishing, as far as possible, 

 these aberrations. In this, however, they were not successful, for coloured 

 images were still produced. Their lenses were constructed of the same kind 

 of material ; and it was afterwards found that in order that lenses might pre- 

 sent the object uncoloured, or be what is called achromatic (, privative, and 

 %f afiit, colour), it was necessary to use glasses of two different densities. 

 Achromatic lenses, or such as are nearly free from aberration, are constructed 

 by placing together glasses of different dispersive powers, and of different forms. 

 The usual achromatic consists of a double-convex lens, made of plate or crown- 

 glass, and a plano-concave, made of flint-glass, fitted accurately to it, and 

 cemented by Canada balsam. Sometimes three lenses are used, the middle one 

 being double-concave, and in that case they cannot be cemented. 



1193. Microscopes 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 trans- 

 parent, by light transmitted from below. This instrument, when used with 

 single lenses or doublets, is the best for ordinary botanical investigations, more 

 especially for dissections. The combination of three lenses approaches too near 

 the object to be easily used. A very high power may be obtained by means or' 

 the lenses termed Coddington's and by doublets formed of plano-convex glasses. 

 The chief objections to the simple microscope are the fatigue attendant on long- 

 continued investigations, and the small field of view. In the simple microscope, 

 glasses of the following focal lengths may be employed viz., 1^ inch, f , *, $ ; 

 and, if very minute objects are to be examined, of -j^, ^ 5 , or $ of an inch. 



1 194. In the Compound microscQpe there are two sets of lenses, the one 

 called the object-glass or objective, the other the eye-piece or ocular. The first 

 receives the rays from the object, and bringing them to new foci, forms an image, 

 which the second treats as an original object, and magnifies it just as the single 

 microscope magnified the object itself. In the construction of the object- 

 glasses, great care is taken to render them achromatic. Those made by the 

 most eminent London makers consist of two or three compound lenses, which 

 cannot be used separately, but are fixed together in a tube. In the case of 

 high powers, the object-glasses are also provided with an adjustment for the 

 thickness of the glass covering the object to be viewed. The eye-piece, 

 also, must be so formed as to be free from error. That used is called Huyghen's, 

 and consists of two plano-convex lenses with their plane sides toward the eye, 

 and placed at a distance apart equal to half the sum of their focal lengths, \\itli 

 a diaphragm placed midway between the lenses. In this eye-piece, the lens 

 next the eye is called the eye-glass, the other the field-glass. The eye pieces 

 supplied with the best microscopes are usually three ; and they are so con- 

 structed, that, with each of the object-glasses, they give a certain amplifi- 

 cation of the object, the powers being in the proportion of 1, 2, and 3, or 1, U, 

 and 2. In the best microscopes there is also an achromatic condenser or 



