NATURAL PHILOSOPHY. 221 



IMPROVEMENTS IN LENSES AND REFLECTORS. 



It is well known that Buffon, desirous of repeating the experiment of 

 Archimedes, Avith a burning glass, endeavored to construct a lens of water, 

 of large diameter. Two plates of glass of great thickness were curved by 

 the heat of a concave metallic plate, worked and polished, and then fitted 

 together with a border of metal and filled with distilled water. Buffon thus 

 made a lens one meter in diameter and of great power. But he pursued it 

 no further, because of the difficulty of the work, and the enormous expense 

 of polishing the rough surface of the glass, the material being also rendered 

 brittle by the second heating. Since then in England, and more lately in 

 France, there have been attempts to blow a glass lens in a mould of metal 

 made in two halves. But the result has been imperfect, the glass uneven, 

 giving no distinct focus. 



There has however been a recent improvement by Messieurs Lemolt and 

 Robert, which is of great importance. It consists in using for constructing 

 the lenses, a circular plate of glass, and a section from a sphere blown with 

 great care, applying this to the plate and closing them together in a circle 

 of metal, and putting water between, or some other transparent liquid. It 

 forms a plano-convex lens, which may be economically made, and has the 

 purity and perfection nearly, without the cost, of lenses of massive glass. 

 Lemolt and Robert have also made improvements in reflectors, employing 

 sections of glass more or less concave, cut from a sphere, in the same way 

 as above mentioned, and having on the convex part a rich plating of silver 

 fz-om electric deposition. These reflectors can be cheaply made and require 

 little care. Lenses and reflectors of this kind have been used on the rail- 

 roads of Paris. By combining the two, a new kind of lamp has been con- 

 structed, giving results of unexpected brilliancy, which have already been 

 brought into use on board ships and at the entrances of ports as well as on 

 rail-roads. A water lens thirty-eight centimeters will send its rays to a dis- 

 tance of twenty kilometers along a railway, producing the effect of a light- 

 house light of the second order. Sillimaii's Journal. 



SIMPLE METHOD OF DETERMINING THE FOCAL LENGTH OF SMALL 



CONVEX LENSES. 



The following is a communication recently made to the Royal (G. B.j 

 Astrominical Society, bv the Rev. T. "\V. Webb. 



* ' / 



The determination of the focal length of a small convex lens is a matter 

 of considerable difficulty, at least in the hands of an amateur. Xot only is 

 the process of direct measurement a delicate and somewhat troublesome one, 

 but the result is not satisfactory, as it is complicated with uncertainties aris- 

 ing from the amount of spherical aberration, which with a large angle of 

 aperture may have a considerable effect ; from the thickness of the lens ; 

 and from the difference of the measure from the centre and from the margin 

 of the posterior surface. These difficulties, it is true, are avoided, as to the 

 usual object of such measurements, by the employment of the dynameter, or 



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