28 



which is made as follows : i dr. of oxide of zinc rubbed down 

 with turpentine, 1 oz. of thick solution of dammar in benzole, and | 

 dr. of gold size. Bell's cement (J. Bell and Co., Oxford Street) 

 is thoroughly reliable and especially useful for sealing up cells con- 

 taining fluids or glycerine. Transparent objects are mounted in 

 various preservative media, such as Canada balsam, glycerine, glycerine 

 jelly, etc. Of these the most useful is Canada balsam, although it is 

 not suitable for all sections or staining methods. It is important to 

 remember that all objects to be mounted in Canada balsam must be 

 quite free from water, and should have undergone " clearing " in 

 either turpentine, oil of cloves, or xylol after staining, whilst objects 

 mounted in glycerine must be first treated with water. We will 

 assume that the object has undergone the necessary preparation and 

 is ready for mounting in Canada balsam. A formula which I can 

 strongly recommend is : Canada balsam (dried) ioo parts, turpentine 

 50 parts, benzole 50 parts. A drop or two of this is placed on the 

 slide by means of the pipette or rod attached to the stopper of the 

 bottle, the specimen placed in it and the cover glass carrying 

 another drop of balsam carefully lowered to exclude air bubbles ; 

 these sometimes give trouble, but slight warming over a spirit lamp 

 and careful pressure with the needle generally dispels them. The 

 specimen is now set aside for several days to set, spring clips being used 

 for maintaining the pressure during hardening. Superfluous balsam 

 may be scraped off when dry, and the slide finally cleaned with a 

 rag dipped in turpentine or xylol. Excavated cells or glass slides, 

 having a depression ground out and polished, are useful for thick 

 objects, and are mounted in a similar way. When mounting in 

 glycerine certain precautions have to be taken in closing the cell. 

 Glycerine is well known to give trouble in this respect, to obviate 

 which all superfluous glycerine should be wiped off with a moist 

 camel-hair brush, and a ring of either shellac varnish or dammar 

 cement laid on. Farrant's medium is an improvement on glycerine 

 as a mountant, and is easily manipulated. It is made as follows : 

 Picked gum arabic, 4 oz.; glycerine, 2 oz.; saturated solution of 

 arsenious acid in water 4 oz. Strain, allow to stand for some weeks, 

 and pour off clear. Glycerine jelly, although much used by the 

 older microscopists, I cannot recommend, as it is the experience of 

 many that specimens mounted by its means have broken down after 

 a few years' keeping. Finished slides should bear a label, giving 

 particulars as to name of object, methods of staining, and mounting 

 and date. Amongst the most practical works on the subject are : 

 "Modern Microscopy," by M. I. Cross and M. J. Cole ; "Methods 

 and Formulae, etc.," by P. W. Squire. 



Photomicrography. 



I had not originally intended to treat of the subject of photo- 

 micrography, this being one to which it is impossible to do justice 



