HARDWICKE'S SCIENCE-GOSSIP. 



79 



and form of the " foundation " for the subsequent 

 treatment. In putting on a sufficient quantity of 

 the foundation varnish to fill up the angle of the 

 cell, it will generally spread irregularly and farther 

 over the slide than is required ; but this can be wiped 

 or driven up to its place by means of the brush 

 handle, which has been cut obliquely to a flattened 

 point, and should be pressed against it whilst the 

 slide is being gently rotated. After this, and while 

 the varnish is quite fresh, the table should be 

 rapidly revolved for a few seconds, to cause it to be 

 evenly distributed around the cell, at the same time 

 that it gives to the surface a high glaze, which 

 imparts to it quite an enamelled appearance. It 

 should now present the outline given in the accom- 

 panying diagram, fig. 34. 



To obtain a secure attachment of the thin glass 

 cover, it is important that its size should not exceed 

 the diameter of the cell itself; but, in fact, it should 

 be a trifle less than the outer edge of the cell wall, 

 leaving a small margin upon the latter to be filled 

 up with the cement, as indicated at a in fig. 35 ; and 

 at this stage no cement, but only the faint edge of 

 the sizing of gum, should project above the glass, 

 as this constitutes a part of the remaining process. 

 It cannot be too strongly impressed upon the 

 reader's attention that it is not the amount of 

 cement spread over the surface of the eover that adds 

 anything to its strength ; but that this latter result 

 is only to be obtained by a thorough embedding of 

 the circumferential edge, as shown in figs. 35 and 36. 

 One of the weakest forms of mounting arises from 

 the edge of the cover projecting beyond and over the 

 outside of the cell/as shown in the section, fig. 37. It 

 will be seen from this that as fast as the soft varnish 

 is applied, it necessarily drains down off the edge of 

 the cover, and leaves this altogether unprotected, 

 as at a, unless indeed such a mass of cement be added 

 as to make it a "bungle," and most unsightly and 

 objectionable. 



We now come to the completing process ; but as 

 the slides in the preceding state will be perfectly 

 secure and not unsightly, they may be put aside 

 until a considerable number shall have been accu- 

 mulated, or they may be completed at once. 



Although gum-dammar is sufficiently hard to 

 resist indentation by the finger-nail, it is yet so 

 remarkably brittle that it may be rubbed to powder 

 between the finger and thumb; hence it will be 

 necessary to add some toughening material, such as 

 the drying oils, to make it durable enough for the 

 outside coatings ; and as the zinc-white used by 

 artists is ground in poppy oil, which is of this 

 nature, this material is well adapted to the purpose, 

 and may be obtained in fourpenny tubes at any 

 artists' colourman's ; but it will be necessary to 

 avoid putting in too much, as this will prevent the 

 varnish from setting hard for a very long time. 

 Should the zinc be very soft, or it be required to 



have a greater degree of opacity in the varnish, this 

 may be obtained by first squeezing out the zinc on 

 to a piece of coarse brown paper, and folding it 

 several times over, and leaving it until the paper 

 shall have absorbed out the greater portion of the 

 oil. The preceding step will be almost indispen- 

 sable with Emerald green, as this colour is mostly too 

 soft to be used in its ordinary state. 



The colours which have been found most suitable 

 are : Emerald green, French ultramarine, scarlet and 

 crimson lake, aureolin, chrome-yellow, Chinese ver- 

 milion, and ivory-black. 



Purples and violets can be made with the lakes 

 and ultramarine, while an infinite variety of the 

 richest transparent greens can be obtained by the 

 ultramarine and aureolin. Transparent colours on a 

 pure white ground give the richest and most re- 

 fined appearance, but for varying the effects it is 

 sometimes desirable to introduce minute propor- 

 tions of opaque colours, and a beautiful China blue 

 may be obtained by adding a little white to ultra- 

 marine, and all may be rendered opaque by the 

 addition of white. 



To prepare the colours. — Having provided 6 or 8 

 one or two-drachm wide-mouthed phials (and if these 

 be cut and polished on the top so as to admit of 

 being closed air-tight with a piece of plate glass, 

 it will be far preferable to corks), half fill each 

 with pure colourless solution of dammar in ben- 

 zole, and then add sufficient of the tube-colour to 

 give the depth of tint required. But before using 

 these colours it will be necessary to give the ground- 

 work-coating of white to receive them, and in 

 doing this a good body of the varnish must be put 

 on and spread a little farther on to the slide, and 

 on to the cover, than will be needed when finished, 

 and then the whirling must be repeated in order to 

 " spin " up a smooth surface. When this has 

 remained for two or three days, and become mo- 

 derately hard, it will then be ready for finishing. 

 Having placed it on the turntable, and made it 

 secure against slipping, take a sharp-pointed knife, 

 or a flat-ended graver, and with this scrape or turn 

 up the edges until smooth and sharp, and perfectly 

 circular, and then wipe away the chips. 



Having previously selected the several colours 

 required, have these at hand, each one with its own 

 brush on a long slender stick, and supported by a 

 clip over the phial, with the hair in the varnish, so 

 that it may be kept in proper working order, and 

 (adding a drop or two of benzole every few minutes 

 to the surface to counteract tackiness), proceed to 

 apply the colours in succession until complete, when 

 the whirling must again be resorted to to make the 

 colours blend and settle down to a flat surface ; and, 

 if _carefully_done, e very l i ne will be perfectlycon- 

 centric with the edges of the cell and Iwith each 

 other. 

 Por 'brushes it will be necessary to select the 



