114 Dr. Griffith on 'preserving Microscopic Objects. 



illuminated (and which is the best way) by the condensation of light 

 by a plano-convex or other lens, the size of the cork or other sup- 

 port for the object is of no consequence. But when the light is first 

 reflected by the mirror and subsequently condensed by a second 

 concave metallic mirror or cup, of course the smaller the object- holder 

 is, the better, because it allows a larger number of rays to be con- 

 densed and to illuminate the object more jierfectly. 



ft. Those which, although viewed as oi)ake objects, are yet pre- 

 served liquid, ai'e kept in cells, which will be presently described ; 

 many animal preparations, injections, &c. are best preserved in this 

 manner. 



Transparent objects. — Different methods from those usually 

 adopted were formerly made use of to prepare these. They were 

 laid upon slides of various kinds, ivory, wood, or glass. The ivory 

 ones (which are scarcely ever now used), of various sizes, had cir- 

 cular apertures turned in them at regular distances ; in these holes 

 small discs of talc were laid; they were prevented from falling through 

 these holes by an elevated rim left at one end, so that the aperture 

 was larger at one end than at the other ; on the other side they were 

 prevented from escaping by the pressure of a small brass ring (cut 

 from off a spiral coil). These, as I have said, are now rarely used, 

 on account of the expense of the turned ivory, the difficulty in ar- 

 ranging the objects nicely, as well as the obvious objections to the 

 talc. The wooden ones are abandoned for nearly the same reasons. 

 They are now generally made of glass. A number of oblong glass 

 slips of the same size* are usually kept already cut, and washed clean 

 with soap and water to free them from grease, &c. Many vegetable 

 preparations can be beautifully preserved by placing them between 

 two of these glass slips, next tying the latter together towards each 

 end by a turn or two of cotton or fine string, then keeping them im- 

 mersed in a wide-mouthed stoppered bottle containing spirits of wine 

 and water. Those only are well kept tlius which are used with low 

 powers of the microscope. When required to be examined they are 

 taken out, drained and wiped ; enough spirit usually remains between 

 the glasses by capillarity to insure their transparency. 



To preserve transparent objects dry, if low powers only be used, 

 they may be laid between two of the above-mentioned slips, allowed 

 perfectly to dry (if previously moist), and the whole slide covered 

 with a doubled piece of pasted coloured paper, excepting two circular 

 or other shaped apertures corresponding to the site of the objects. 



In some cases Avhere the objects are not quite so transparent as we 

 should wish — in, I may say, all cases of the preservation of crystalline 

 bodies, particularly when for examination by polarized light — they 

 may be immersed in Canada balsam. For low powers take two 

 glass slips, having heated one gently over the spirit-lamp (at a great 

 distance however) ; place upon it a small quantity of Canada bal- 

 sam from the end of a piece of stick. Let this perfectly, but slowly, 



* This varies according to the whim of different observers, but the thick- 

 ness is best about that of ordinary window glass. 



