638 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



tlireads of flint or crown glass of various densities are placed concentrically 

 one in the other, and superposed or not at their extremities. These circles 

 or parcels of threads are made of any thickness ; the threads of glass may 

 also be placed vertically. 



" The refraction and dispersive power is also caused t& vary in lenses, 

 the density of which is variable from the centre to the edge, by placing 

 tubes formed of glass of different densities, as flint and crown glass, 

 figs. 27, Oi, h, c, d, e, one inside the other, which are softened by heating 

 and again blown. Other tubes of greater density /, g, Ji, i, are placed inside 

 them, softened, and again blown. Tubes J, h, I are again put in until the 

 whole and the centre are well filled, they are again softened and drawn, 

 and a cylinder is obtained. If drawn with sufiicient rapidity the whole 

 of the concentric cylinders will only form a single cylinder of greater 

 or less thickness, or if the cylinders interposed are sufficiently numerous, 

 a convex lens cut from this cylinder will be deprived of spherical aberration 

 and even of chromatic aberration if the density augments from the edge 

 to the centre, if concave it will be also deprived of aberration, if the density 

 augments from the centre to the edge. These tubes, brought at their 

 extremity to the point of fusion, may be blown, and Microscope lenses 

 will be formed that will be in concentric layers and will be achromatic and 

 without aberration. By bringing them to the point of fusion lenses may 

 in this manner be produced, the outer layers of which will be less dense 

 and have the form and arrangement of layers analogous to those of the 

 crystalline lens of the human eye and will be achromatic. 



"Under certain circumstances concurrently with the plates, circles, 

 bundles, and fluxes, silicates in solution in hydrochloric acid or hydrofluoric 

 acid may be employed diluted with water or combined with other transparent 

 substances either to cause the plates to adhere together or to obtain the 

 required degree of refraction or dispersion. Intermediate layers of crystal- 

 lized boron, sesquichloride of carbon, crystallized or melted silicic acid, 



bichloride of tin, small crystals, and 

 even powder of any kind, principally 

 powdered glass, either colourless or of 

 many colours, obtained by the method 

 of granulation above described, are 

 applied by means of heat and pressure, 

 currents of electricity, and other me- 

 chanical and chemical forces aided by 

 blowing, moulding, and motion. By 

 these means all the required forms and 

 qualities are obtained, so that the re- 

 frangibility, dispersion, transparence, 

 malleability, density, hardness, and elas- 

 ticity of these lenses may be varied." 



Amongst other matters dealt with 

 are a screw guide for sliding tubes, 

 adjusting screws with differential 

 threads for slow or rapid motion, a 

 universal joint to foot with clamp, im- 

 proved stages, &c. 



Bausch and Lomb Optical Go's 

 Trichinoscope. — Another form of this 

 instrument (described Vol. II., 1882, p. 258) is shown in fig. 157, the 

 doublet being replaced by a compound Microscope which is combined with 

 the compressor (described Vol. III., 1885, p. 714). 



Fig. 157. 



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