148 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG.' 114 



for which a low magnifying power suffices; and it is specially important, 

 in the examination of such objects, not to use a lens of shorter focus than is 

 absolutely necessary for discerning the details of the structure; since, the 

 longer the focus of the Objective employed, the less is the indistinctness 

 produced by inequalities of the surface, and the larger, too, may be its 

 aperture, so as to admit a greater quantity of light, to the great improve- 

 ment of the brightness of the image. Objectives of long focus are espe- 

 cially required in Microscopes that are to be used for Educational purposes; 1 

 and an endless variety of ( common objects ' suitable to these may be found 

 by such as will take the trouble to search for them. The mode of bringing 

 Opaque objects under view will differ according to their 'mounting,' and 

 to the manner in which it is desired to illuminate them. If the object 

 be mounted in a ' slide ' of glass or wood, upon a large Opaque surface, the 

 slide must be laid on the stage in the usual manner, and the object brought 

 as nearly as possible into position by the eye alone ( 141). If it be not 

 so mounted it may be simply laid upon the glass Stage-plate, resting 

 against its ledge; and the Diaphragm-plate must then be so turned as to 

 afford it a black background, light being thrown upon it by a Condensing 

 Lens or Bull's-eye placed as in Fig. 114, or (still better) by Beck's Para- 

 bolic Speculum, which gives a far better illumination by diffused daylight 



than can be obtained by any 

 other means yet devised, and 

 which is equally well adapted 

 to lamp-light, when used in 

 combination with the Bull's- 

 eye ( 114). Direct sunlight 

 cannot be employed without 

 the production of an injuri- 

 ous glare, and the risk of 

 burning the object; but the 

 sunlight reflected from a 

 bright cloud is the best light 

 possible. When a Condens- 

 ing Lens is used, it should 

 always be placed at right 

 angles to the direction of the 

 illuminating rays, and at a 

 distance from the object 

 which will be determined by 



the size of the Surface to be il- 

 luminated and by the kind of 



light required. If the magnifying power employed be high, and the 

 . field of view be consequently limited, it will be desirable so to adjust the 

 lens as to bring the cone of rays to a point upon the part of the object 

 under examination; and this adjustment can only be rightly made whilst 

 the object is kept in view under the Microscope, the condenser being 

 moved in various modes until that position has been found for it in 

 which it gives the best light. If, on the other hand, the power be low, 

 and it be desired to spread the light equably over a large field, the Con- 

 denser should be placed either within or beyond its focal distance; and 



1 The makers of Educational Microscopes supply at a small cost, single (triplet) 

 combinations of 3 inches, 2 inches, or 1^ inch focus, or dividing combinations of 

 half inch and 1 inch, 1 inch and 2 inchs, or 1^ inch and 3 inches, which are quite 

 adequate for ordinary requirements. 



Arrangement of Microscope for Opaque Objects. 



