J J. 



HARDWICKE'S SCIENCE-GOSSIP. 



cability and its results, and it is satisfactory to find ' 

 that it has proved to be all that could be desired. 



In the days of the old monocular microscopes, 

 thin cells and translucent objects were chiefly 

 required ; but since the introduction of the bin- 

 ocular, deeper cells and both wet and dry mountings 

 of considerable size have to be provided, and it is 

 no very easy matter to devise means of resisting or 

 compensating expansion and contraction in these 

 more bulky receptacles ; for if the cement be too 

 hard, the air will obtain access, and if too soft or 

 yielding, the cement itself will be gradually drawn 

 in and become equally disfiguring, a defect speci- 

 ally pertaining to the gold size when mixed with 

 cement; and the desideratum was to find some 

 substance which should retain a slight degree of 

 tenacity without the same disposition to spread or 

 run, and this merit Ackland's varnish appears suf- 

 ficiently to possess. 



Eor cells requiring 'to be attached to the glass 

 there are three substances, namely, glass, metal, 

 and paper, which are most to be relied on : the two 

 former may be fixed clown with marine glue, but 

 there are innumerable objects for which properly 

 prepared paper rings or cells are admirably adapted, 

 as being both convenient and economical, and which 

 may be fixed down with less trouble by cementingj 

 and be equally durable and safe. Ebonite cells 

 were at one time strongly recommended ; but these 

 have been discarded, owing to the almost impossi- 

 bility of attaching them to the glass in a sufficiently 

 secure manner. 



For the method of making these paper rings and 

 cells, the reader is referred to p. 16 of the same 

 volume (1874). 



Requiring to mount a dry preparation in a deep 

 cell, and having the latter securely attached to the 

 glass slide, place the object in its centre, and take 

 care that the whole be perfectly dry before pro- 

 ceeding, and close it up ; but when this has been 

 attained, and the slide properly centred upon the 

 turntable, form a ring of the cement upon the edge 

 of the cell, — letting it stand up as a prominent ridge 

 rather than be spread evenly over the surface. Now 

 pick out a proper-sized cover, and by the time this 

 has been sufficiently cleaned and ready for use, the 

 cement will be in a fit state to receive it, when it 

 may be dropped into its place, and slightly pressed 

 down to make it adhere enough to avoid shifting 

 when the slide is handled. As the cement will 

 have become too hard to be pressed into shape with- 

 out warmth, heat a plain glass slide as hot as it can 

 be held, and then placing it upon the cover nip the 

 two together between the finger and thumb until 

 the whole of the cement forms one entire black 

 circle without any interrupting fissures, which may 

 be known by their being lighter in colour. 



By attaching cells or covers in this way by means 

 of heat applied to a half-set cement, two advantages 



are derived ; first, the heat necessary to soften the 

 cement secures its firm adhesion to 'the glass, and, 

 next, from its being in a half-dried state, it is, .when 

 cool, sufficiently tenacious to resist the solvent 

 powers of additional coats of varnish or cement, and 

 is thus deprived of the tendency to be sucked in by 

 any exhaustion of the cell's atmosphere. To insure 

 security it is always better to give one outside coat- 

 ing of the thin varnish, embracing both a small 

 portion of the slide and of the upper surface of the 

 cover, before setting it aside to complete the drying_ 

 In using this asphalt varnish there are two 

 points requiring attention ; in the first place it must 

 not be applied in too thick a state, as under these 

 circumstances it does not work with the proper 

 freedom, and in the second place it must not have 

 fresh benzole added at the time of using, for it will 

 then be found to be knotty or lumpy. The best 

 plan is to have several one-drachm vials with it, suffi- 

 ciently thin for use, and when one has become 

 unworkable, to add benzole, and put it aside for 

 another, until its turn shall have come round again. 

 In the putting up of snake objects in dry cells, 

 one of the greatest troubles experienced by careful 

 mounters lies in the extreme difficulty of securing 

 the absence of dust, and other extraneous matter 

 from the interior of the cells and the underside of 

 the covers. There is one circumstance affecting the 

 cause of this disposition to accumulate foreign 

 substances upon the surface of glass which, if 

 generally known, is very commonly neglected. Let 

 a stick of sealing-wax be rubbed briskly on the 

 coat-sleeve for a second or two, and then be pre- 

 sented to any light substances, such as small pieces 

 of paper, feathers, or cotton threads, &c, and these 

 will be seen to instantly start up, and adhere to its 

 surface. Now as glass is also an electric, and can 

 be excited in the same way, it stands to reason that 

 when a cover is rubbed either w ; th silk, linen, 

 leather, or any similar substance, it will be made to 

 attract dust, and other bodies floating in the air of 

 the room, or resting within reach of its influence. 

 Acting upon this principle, and after trying nu- 

 merous plans, the following has been adopted as 

 proving the most successful. 



Take an eighth part of a sheet of stout blotting- 

 paper, and roll it up firmly into a cylinder about as 

 thick and as long as an ordinary cedar pencil, binding 

 it round with a piece of plain paper, gummed or 

 pasted at the last coil, to keep it compact. When 

 dry, cut off one end square and the other obliquely, 

 so as to present a flat point or edge ; after which rub 

 the cut surfaces on a rather smooth file, or scrape 

 them with the edge of a knife, so as to render them 

 soft and fluffy, like the pile on the face of velvet. 



Next, procure a flat cork or bung, about two 

 inches in diameter, and over one of its surfaces 

 stretch a piece of soft, thin wash-leather, tying it 

 around the edge, so as to form a table for receiving 



