MOUNTING OBJECTS IN CANADA BALSAM. 211 



objects, however — such as Diatoms and Polycystina — are best 

 mounted on small disks of thin blackened card attached to Glass 

 slides ; being protected either by Ring-cells (§ 169) of Glass, Metal, 

 or Vulcanite,* or by perforated disks cut with punches of suitable 

 size out of cardboard or kid-leather, which, having been repeatedly 

 brushed over with Liquid Glue, are attached to the slide, and have 

 their covers affixed to them with the same material. 



157. Mounting Objects in Canada Balsam. — This method of 

 mounting is suitable to a very large proportion of those Objects 

 which are to be viewed by transmitted light, and whose texture is 

 not affected by the loss of the aqueous fluid they may contain ; 

 and it has many advantages over the mounting of the like objects 

 dry. For, in the first place, as it fills-up the little inequalities of 

 their surface, even where it does not actually penetrate their sub- 

 stance, it increases their transparence by doing-away with irre- 

 gular refractions of the light in its way through them, and gives 

 them the- aspect of perfect smoothness ; this is well seen in the 

 case of sections of Shell, &c, which, when thus mounted, do not 

 require a high polish (§ 140). But, secondly, where the structure, 

 although itself hard, is penetrated by internal vacuities, the Balsam, 

 by filling these, prevents that obscuration resulting from the inter- 

 position of air-spaces, and from additional internal surfaces of 

 reflection, by which the transmitted rays are distorted, and a large 

 proportion of them lost : this is well seen in the case of the Fora- 

 minifera, and of sections of the 'test' and 'spines' of Echinida, 

 whose intimate structure can be far better made-out when they 

 are thus mounted, than when mounted dry, although their sub- 

 stance is (for the most part at least) itself so dense, that the 

 balsam cannot be imagined to penetrate it ; and likewise with dry 

 Vegetable preparations, which are perhaps also affected in the 

 manner to be next described. Thirdly, there are very many struc- 

 tures of great interest to the Microscopist, whose appearance is 

 extraordinarily improved by this method of mounting, in conse- 

 quence of a specific effect which the Balsam has in combining (so 

 to speak) with their component elements, so as to render them far 

 more transparent than before : this effect is seen in the case of all 

 dry preparations of Insect-structure, especially of such as consist of 

 their hard external tegument or of parts derived from this ; also in 

 the various Horny tissues (hairs, hoof, horn, &c.) of the higher 

 animals; and likewise in many organized substances, both recent 

 and fossil, which are penetrated by Calcareous matter in an amor- 

 phous condition. — Besides these advantages, the mounting of ob- 

 jects in Canada balsam affords one of the easiest methods of fixing 



* Ring-Cells cut in a lathe from Gutta-percha tubing have beeD pro- 

 posed for this purpose ; but they do not adhere permanently to glass ; 

 and Cells of Vulcanite made in the same manner are greatly to be pre- 

 ferred. Cells cut off from Pasteboard tubing may also be employed, if 

 treated with Liquid Glue as, mentioned above. 



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