July 18, 1884.] 



♦ KNOVs^LEDGE ♦ 



47 



We mean as a magnifying glass. Here the object (suppose 

 a tiny arrow) k w is placed just wimin the principal focua 

 of the lens, and the rays from it being cdusid to converge 

 to the eye at E seem to come troiu a much larger arrow 

 A W. The image thus perceived is, of course, a virtual 

 one only, as contradistinguished from the real image formed 

 by the camera obscura, as just described. It is needless to 

 discuss tlie passage of light through a concave lens here, 

 as we are not writing a treatise on optics. It may suffice 

 to say that, mutatis mutandis, its action is comparable with 

 that of a convex mirror (p. 437) — i.e., it renders rajs of 



light passing through it more divergent, just as is the 

 action of a convex lens with the effect produced by a con- 

 cave mirror, in causing them to converge more. A convex 

 lens, too, fulfils another function — that of grasping a number 

 of raysofliglit, which varies as the square of its aperture. A 

 "burning-glass" illustrates this property admirably. Let 

 us suppose that we have a convex lens of 3 in. in diameter 

 and of 6 in. focus. Then the image of the sun formed in 

 that focus will be only 00558 in. in diameter. But this 

 image is formed by all the rays incident on the 3-inch 

 aperture of the lens ; and, as we know that the area of 

 circles vary as the squares of their diameters, we have only 

 to divide 3- by 0-0558" to see at once how relatively enor- 

 mous must be the concentration of light and its concomitant 

 heat in the sun's image projected by a lens of our assumed 

 size and focus. When such a lens is made of sufficient 

 dimensions, its effect is astounding. Parker constructed a 

 flint lens of 32 in. in diameter, of 6 ft. S iu. focus ; using 

 together with it a second one of 13 in. diameter and 29 in. 

 focus to further concentrate the converging rays. These 

 lenses were so arranged that their combined focal length 

 was 5 ft. 3 in. With this combination, 10 grains of slate 

 were melted in 2 sec, and 10 grains of pure platinum in 

 3 sec. Nay, even so utterly refractory a substance as 

 rottenstone disappeared in vapour in 1 min. 20 sec. under 

 the inconceivable intensity of the heat thus generated ! 

 Similar experiments were made a few years ago with a lens 

 built up of segments, at the Crystal Palace, under which a 

 halfpenny was vaporised in a comparatively few seconds. 



{To be continued.) 



ELECTRO-PLATING. 



VIII. 

 By W. Slingo. 



AS was intimated a fortnight since, a deposit of copper, 

 if it is allowed to assume any considerable thick- 

 ness, loses very materially in definition, so far, that is, 

 as concerns the outer or exposed surface. Although the 

 deposit may often, under the most advantageous circum- 

 stances, be made to retain the general features and beauties 

 of the mould, such a result must not be habitually looked 



for ; but where plating is preferable to typing, the necessity 

 for allowing only a tliiu deposit must not be overlooked. 

 It is far better, where a good and substantial deposit is 

 required, to obtain a type. Even, however, were it other- 

 wise, the mere jiroduction is in itself interesting and in- 

 structive, and is certainly a stepping-stone to higher and 

 better achievements. 



It is in the nature of things that casts should be more 

 or less undercut, and as it is my purpose here to deal with 

 the problem of copying models in high relief, it will pro- 

 balily be better for us to concentrate our attention upon 

 sou:e particular form. Let us imagine that we desire a 

 representation in copper of a small bust or statue, classical 

 or otherwise. Now, it will be apparent on the briefest 

 reflection that such a model cannot be copied in plaster or 

 any other of the rigid materials used in making moulds 

 from comparatively flat models such as coins or medals. 

 Were our model a metal bust we could, of course, take a 

 copy of it by enveloping it in a bath of wax or some other 

 acid-proof substance, and then dissolve out the metal by 

 means of acid, but that plan would rarely be desirable, 

 more particularly if the metal were valuable, or of an 

 obstinate nature, requiting nitric acid to dissolve it. 



A better way, by far, is to use what is known as an 

 elastic mould. A brief reference was made to such a mate- 

 rial in the fifth of this series of articles (Knowledge, No. 

 134). It was there said that such a mould could be made 

 from glue and treacle. If the figure to be copied is smEill, 

 two pounds of the finest glue is broken up into small 

 pieces and soaked in cold water until it becomes quite soft. 

 Any water that remains unabsorbed is poured off, and the 

 gelatinous mass is then placed in a glue-pot with half-a- 

 pound of treacle, and heated to nearly 100' C. (boiling point 

 of water). To comply with this does not, of course, require 

 a thermometer. A glue-pot, properly speaking, consists of 

 two pots, one inside the other, the inner one containing 

 the glue and the outer one containing water. The two so 

 fitted are placed on the fire, and, as doubtless most people 

 are aware, the most intense fire is incapable of raising the 

 glue to a higher temperature than that of the water through 

 which the heat is transmitted, and that water, as water, 

 cannot, above the sea-level, be raised to a higher tempera- 

 ture than 100° C. (or 212° F.), any heat passing into the 

 water after such a temperature has been attained being 

 absorbed in the conversion of the water into steam. So 

 long, then, as there is water in the outer vessel, the tem- 

 perature of the inner one cannot exceed the boiling-point 

 of water. 



The mixture of glue and treacle during the process of 

 heating should be thoroughly stirred, so as to ensure a 

 uniform resultant compound. An ounce or so of beeswax 

 may be added with advantage. 



Supposing, now, that the bust or figure to be copied is 

 made of plaster, its surface must manifestly be well pre- 

 pared so as to make it non-porous, otherwise the mixture 

 will get into the interstices and render it impossible to eflfect 

 a separation. The plaster, therefore, should be stood or 

 laid in a shallow di.sh containing oil and thoroughly satu- 

 rated. If a metal or other " solid " model, it requires 

 well oiling to prevent the mixture adhering to it. A 

 vessel, such as a jar, or a pail if the figure is a large one, is 

 then procured and its interior well oiled. Presuming the 

 model to be hollow, it is filled with sand, in order 

 to increase its weight. It is then placed head 

 downwards in the jar, a mark being made on the 

 outside to indicate the position of the back of the 

 figure. The mixture being warm, is then poured in, but 

 not too rapidly to prevent the escape of any air bubbles 

 that might collect in one or other of the various crevices of 



