ARRANGEMENT FOR OPAQUE OBJECTS. 



161 



duced 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 

 improvement of the brightness of the image. Objectives of long 

 focus are especially required in Microscopes that are to be used for 

 Educational purposes ; since it is most important that the young 

 should be trained in a knowledge of the wonders and beauties of 

 the familiar objects around them, and of these an endless variety 

 may be found by such as will take the trouble to search for them, 

 which can thus be viewed with great facility.* 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 (§ 115). 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 



Fig. 89. 



Arrangement of Microscope for Opaque Objects. 



background, light being thrown upon it by a Condensing Lens or 

 Bull's-eye placed as in Fig. 89, or (still better) by Beck's Parabolic 

 Specidum, which gives a far better illumination by diffused daylight 

 than can be obtained by any other means yet devised, and which is 



* The makers of Educational Microscopes (p. 64, note) supply p.t a 

 small cost single (triplet) combinations of 3 inches, 2 inches, l£ inches 

 or 1-inch focus, which are quite adequate for ordinary requirements. 



M 



