DRY-MOUNTING OPAQUE OBJECTS. 209 



with a central aperture of from 3 to 5-Sths of an inch ; to cover this 

 aperture "with a ' square ' or ' round ' of thin glass of sufficient size 

 to project considerably beyond it ; to lay the object upon this glass, 

 and to protect it with a cover of rather smaller size, which should 

 be fastened down all round by varnish to prevent the entrance of 

 moisture ; and finally to secure both glasses to the wooden slide, by 

 gumming down over them a piece of paper with a perforation of 

 the same size as that of the slide itself. 



155. For dry-mounting Opaque objects, the method adopted 

 must vary with the mode in which the object is to be illuminated. 

 If a Side -Condenser or Parabolic Reflector is to be employed, which 

 is the most appropriate method for the great majority of objects, 

 the whole slide may be opaque ; and the following simple plan 

 devised by the Author (whose entire collection of Foraminifera is 

 thus mounted) will be found to afford peculiar conveniences. Let 

 there be provided a Wooden slide of the kind just described, a 

 piece of card of the same dimensions, and a piece of dead-black 

 paper, rather larger than the aperture of the slide, if a dark 

 mounting be desired, -which is preferable for most objects : this 

 piece of paper is to be gummed to the middle of the card, and then, 

 some stiff gum having been previously spread over one side of the 

 slide (care being taken that there is no superfluity of it imme- 

 diately around the aperture), this is to be laid down upon the 

 card, and subjected to pressure.* An extremely neat 'cell' will 

 thus be formed for the reception of the object (Fig. 9S), the depth 



Fig. 98. 



Wooden Slide for Opaque Objects. 



of which -will be determined by the thickness of the slide, and the 

 diameter by the size of the perforation ; and it will be found con- 

 venient to provide slides of various thicknesses, with apertures of 

 different sizes. The Cell should always be deep enough for its wall 

 to rise above the object : but, on the other hand, it should not be 

 too deep for its walls to interfere with the oblique incidence of the 

 light upon any object that may be near its periphery. The Object, 

 if flat or small, may be attached by ordinary Grum-mucilage ;+ if, 



* It will be found a very convenient plan to prepare a large number of 

 such slides at once : and this may be done in a marvellously sbort time, 

 if the slips of card have been previously cut to the exact size in a book- 

 binder's press. The slides, when put together, should be placed in pairs, 

 back to back ; and every pair should have each of its ends embraced by 

 a Spring-Press (Fig. 101) until dry. 



t It will be found very advantageous for almost every purpose to add 



P 



