MISCELLANEOUS HINTS. 487 



upon the coloured papery but the most satisfactory method of 

 proceeding, is to paste a piece of white paper upon one end of 

 the slide, and to punch out a circular or other hole in the 

 coloured paper that is pasted over it; by these means there is 

 less risk of the label being lost, as it is doubly protected. 

 When the slides are not papered, the name should be written 

 on them by the diamond described at page 261 ; it will be 

 often advisable, when fluid is used, to put down the name of 

 it, and the date when the preparation was mounted. For the 

 sake of cataloguing the slides, the opposite end to that having 

 the name should be employed for the purpose. 



Mr. C. Brooke's Method of Viewing Opaque Objects under the 

 High Powers. — A truncated parabolic mirror, now known as 

 Wenham's reflector, with a dark well in the centre, is placed 

 under the object, and illuminated by a parallel pencil of rays, 

 obtained by placing a combination of two plano-convex lenses 

 underneath, and concentric with the mirror, the lamp being 

 in the principal focus of the combination. 



A smaU plane mirror is attached to the object-glass, the 

 surface of which is level with or very little below the external 

 surface of the object-glass. The rays of hght converging to 

 the focus of the parabolic mirror, being received on the plane 

 mirror, are thrown on the object. By this arrangement, all 

 the rays that subtend any angle from that of the object-glass 

 up to about 170° are made available for the illumination of 

 the object. When the one-eighth and one-twelfth object- 

 glasses are used, it is necessary that the object should be 

 mounted on a small surface, without any pits or depressions ; a 

 truncated cone of cork, wood, or ivory, with an appropriate 

 holder, may be used. 



For a finder it is convenient to have a plane mirror attached 

 to a cap fitting over the one-inch object-glass, so that the 

 surface of the mirror may be about one-tenth of an inch above 

 the focus, the hole in the centre being just large enough 

 to admit all the rays that can enter the object-glass. 



In order to obtain the best definition of minute objects in 

 fluid under the high powers, it is necessary to have thin glass 

 under the cell, as well as over it. For this purpose, Mr. 



