110 THE VICTORIAN NATURALIST. 



refractive index than Canada balsam alone. My experience 

 of it is, however, brief, for I have only used it in the preparation 

 of slides in illustration of this paper. Various other media have 

 been tried from time to time, but all have so far proved either 

 difficult to manipulate, or, what is of more importance still, are not 

 permanently reliable. Diatoms, I need hardly say, should always 

 be mounted on the cover, no matter what the medium used may 

 be. The finer kinds scarcely require any further attachment than 

 what is provided by drying them upon the cover glass over a 

 spirit or other lamp. The larger forms require that this be first 

 smeared with a very weak solution of gelatine by means of a 

 needle. The exact strength of this solution can only be 

 determined by experiment. This applies also to the preparation 

 of selected slides. The mode of procedure I adopt with " spread 

 slides " is to take a slip, breath upon it, and then place thereon 

 one or more covers. Upon these I drop from a pipette the 

 water containing the diatoms, distributing the latter evenly over 

 the surface by means of a needle, keeping the slide meanwhile 

 perfectly horizontal and steady. They may be left to dry spon- 

 taneously or heat may be applied. The covers are then taken 

 up in a forceps and placed in position on a slide, upon which 

 there has previously been placed a sufficiently large drop of 

 Canada balsam. No pressure must be applied to the cover, as if 

 the balsam is of the proper consistency it will flow to the edge of 

 the cover without this. Nor is it desirable to smear the slide with 

 balsam unnecessarily, nor yet is it a difficult matter to guess or 

 otherwise find its centre to within an inch or so. These are, of 

 course, merely matters of detail, but are nevertheless worthy of 

 attention. The slide may now be put away to dry, secured from 

 dust, &c. Should any bubbles be present in the balsam at the 

 time of mounting, they will generally make their way in course of 

 time to the margin of the cover, or otherwise disappear. After 

 the slide is thoroughly dry, it will contribute to its neatness and 

 permanence if a ring of some cement or other be run round the 

 cover by means of a turntable. This, I know, is a practice that 

 is sometimes "universally condemned" as savouring of the amateur 

 and dilettante — a species of reproach an otherwise well-mounted 

 slide will generally survive. 



A word or two as to selected or picked slides. This method 

 of mounting enables us to isolate certain species for special 

 examination or naming, or for the purpose of reducing the 

 number of slides in a series, or even for effect. It is easily 

 accomplished, requiring only practice, patience, and a steady 

 hand. To effect this I use a mounted bristle obtained from a well- 

 worn clothes brush. Formerly, on the recommendation of Dr. 

 Carpenter, I used a cat's whisker, but as the cat was not always 

 on hand or agreeable, I found the brush hairs served the purpose 



