194 



HARDWICKE'S SCIENCE-GOSSIP. 



C. G. Leland, page 62, the following recipe for 

 making tracing-paper, which can be reconverted into 

 ordinary opaque drawing-paper after drawing upon 

 it:— 



If one volume of castor-oil be dissolved in two or 

 three (I prefer three) volumes of spirit of wine, the 

 mixture will render paper immersed in it transparent, 

 and the spirit rapidly evaporating, the paper will 

 become fit for use in a very few minutes. (I find 

 two or three hours necessary to dry it.) If the paper 

 be afterwards placed in spirits of wine, the oil is 

 dissolved out, and the paper presents its original 

 appearance. 



I find the recipe to be quite correct, and think that 

 many readers of this paper will find it useful. 



The illumination I make use of is that of Swan's 

 Special Microscopical Incandescent Electric Light, 

 of which more in last month's number. The full 

 power of the battery should be put on, so as to give 

 a dazzling light if the object be viewed with the 

 microscope eye-piece, otherwise a clear image will 

 not be cast on the paper. As, however, most readers 

 of Science-Gossip have not the electric light, perhaps 

 a few words as to the use of daylight would be use- 

 ful. The dazzling light reflected directly from the 

 sun, gives the most distinct image. Should the heat 

 of the rays be feared, it might be cut off by interposing 

 a cell containing a solution of ammonio-sulphate of 

 copper between the light and the mirror, although, 

 not having tried this, I am afraid the light would be 

 much diminished. The tracing-paper must* be ex- 

 ternally in darkness. This may be secured by closing 

 the shutters and drawing blinds and curtains so as to 

 exclude all light but that on the mirror, or by cover- 

 ing one's head with a dark-cloth like a photographer. 

 Probably the most convenient plan would be to cut a 

 hole large enough to admit the tube of the microscope, 

 including the adjustment milled-head in a dark tent. 

 If this were done the camera-box might be dispensed 

 with, and the image might be thrown on to tracing- 

 paper stretched over a framed plate of glass placed at 

 any convenient distance. 



A dark tent, which would be easy to make, is de- 

 scribed by Mr. J. C. Leake in CasselPs " Popular 

 Recreator," vol. ii. page 227. The following is an 

 extract from his remarks : — 



" The first thing required will be a shallow box or 

 tray of wood which will form the bottom of the tent. 

 This should be made of dry pine about half or three- 

 quarters of an inch in thickness. For a nine-by- 

 seven (photographic) plate the base should be about 

 two feet by eighteen inches. To the outer edges of 

 this board should be screwed a wooden rim, about 

 two inches deep (and one inch thick). This will form 

 a capital operating table of convenient size. The top 

 will consist of a light board, say half an inch in thick- 

 ness, and of exactly the same size as the base ; round 

 this must be screwed a fillet of wood, so placed as to 

 fit into the tray which forms the base of the tent. 



" The next thing will be the covering, which should 

 consist of two thicknesses of black "twill" (also 

 yellow calico if used for photography). This 

 covering should be secured at the edges by small 

 tacks to the inner side of the bottom tray, and the 

 inner side of the top fillet, taking care to nail it very 

 closely, so as to exclude light, and make it very 

 secure. The covering material should be made 

 to extend along the back of the tent, over the two 

 ends, and for six inches each way along the front. 

 (The length way of the boards forms front and back, 

 the ends form sides.) Each thickness of covering 

 material will therefore require to be two yards in 

 length and the edges in front of the tent will require 

 to be neatly stitched, so as to unite the thicknesses. 

 In order to ensure the tent standing square when 

 erected the lining should be fixed both at the top and 

 bottom, quite parallel with the base and cover. 



" In order to stretch the lining and erect the tent, 

 two stout rods will be required, which should be 

 rather longer than the height of the tent, when the 

 top is raised to the extent allowed by the lining. If 

 these rods are inserted at the ends of the tent, and 

 pushed firmly up, so as to stretch the covering out 

 tightly, a box will be formed ; which, if the work has 

 been well done, will be almost as firm and rigid as if 

 made of wood. 



'* The next thing will be to provide a curtain 

 which may cover the opening left in the front of the 

 tent. For this purpose the same black material may 

 be employed, and it should be large enough to reach 

 some little distance round the edge to the ends of the 

 tent and to fall quite loosely. 



"This curtain should now be secured to the top 

 by strings of tape inserted in small eyes placed upon 

 the cover of the tent, and to the sides by strings of 

 the same material, fastened both upon the curtain and 

 the sides of the tent. The curtain (being intended to 

 wrap round the operator and exclude light) should be 

 made very full, and long enough to fall at least two 

 feet below the bottom of the tent. The whole tent, 

 when folded up, will be enclosed in a flat box about 

 three inches in thickness." 



For use with the microscope probably the best 

 plan would be to make the base board six inches 

 wider, say two feet by two feet. Then the tent 

 might be made of the size above described, and a 

 ledge two feet long and six inches wide would be left 

 outside at the back, at the opposite side of the tent to 

 the operator. Two holes might be made in the twill 

 at the back, one of them provided with a tape to 

 draw it up tightly round the lower end of the 

 microscope limb, the other having a circular brass 

 curtain-ring stitched into it to admit the light from 

 the mirror. If thought desirable, a broader ledge to 

 hold a lamp might be left. Such a tent would be 

 convenient for ordinary work with the microscope, as 

 I have seen somewhere pointed out. 



If the eye-piece be removed from the microscope 



