4 
REPORT—1854. 
a very simple disposition to the micrometric part of the apparatus, so that when the 
telescope which moves the screw is removed, it readily forms a very delicate sphere- 
meter. 
By operating in this manner, but with a very imperfect apparatus, I found, for the 
indices of refraction of the ordinary ray by plates of quartz of 7 to 8 millimetres in 
thicknesa,the numbers 1 '5441,1 '5473,1 *3490 and 1-5541, which correspond respectively 
to the rays D, E,F, G. These numbers do not differ from those given by M.Rudbcrj 
by more than ^—jth* at most. 
This process may be advantageously employed in researches relative to the indie** 
of refraction of liquids ; the mode of operation is so simple that I may dispense with 
entering into details upon this subject. 
Description of a Photometer exhibited to the British Association for the 
Advancement of Science. By Dr, Felix Bernard, Bordeaux. 
The photometer which I have the honour to bring before the Association ha* ht'" 
exhibited by M, Arago to the Academy of Sciences, at its meeting ot the vtn 1 
1853. This instrument, which is principally intended for tlic study of * 
rency of bodies, may be very advantageously employed in a great numbero p ^ 
metric investigations, regarding the illuminating power of luminous o 1 > ^ 
diffusion of light, and its reflexion from the surfaces of bodies, &e. . s 1,1 
particularly the arrangement which I have adopted for ascertaining the 
exercised upon light by the action of transparent media. -hedwiti 
In this case the anterior portion of the instrument presents u tube m „ | ^ 
two diaphragms, through which the luminous rays, previously retiderea p rf 
menus of a lens, penetrate into the apparatus. At their entrance in ' , 
the instrument, the rays full perpendicularly upon the middle ot 0 f«|ii,* 
double prism, formed by the union of two equal, rectangular, isosceles P™" •' ,u 
two faces are in the same plane. The bundle formed by these rays ‘ ' f; 
edge of the right dihedral angle of the hypothemtses, is divided into ^ ( ] wl ( 
bundles, which are entirely reflected in opposite directions perpeudicu 
the incident bundle. At a few centimetres from the axis there are t ^ ^ 
prisms, by meant of which the rays ore reflected in a direction paia | ar jjei !l 
farther on the new bundles traverse some Nicole prisms which serve rin J, e t r ic»l 
analyse the light, and by a second arrangement of reflecting prisms sy lur t,-: 
the first, these bundles, after passing through the Nicols prisms, ar o ^ ni , 
towards one another and reflected upon the hypothenuses of a sew bun( |!e 
so as, by their reunion, to reconstitute the primitive bundle. - ve3 a t the 
traverses a tube forming the ocular portinu of the instrument, ana hi 
as though it had never undergone any deviation. „ - om » r0 ence of |V 
At one extremity of the ocular tube and close to the surface ?! 1 emi t firC11 ii 
double reflecting prism, there is a diaphragm, the aperture of which* 
image upon this surface; this is magnified by a lens. circles by 
The rotation of the analysers is measured upon two vertical sem 
alidades furnished with verniers. . . 
ie apparatus being arranged, that is to say, the extinction bMg ' ^ 
the verniers are. at zero, the intensity of each half of the circular iiavgV^ !m t 
the eye varies in proportion to the square of the sinus of the ang “ m 
principal section of llic analyser ; and consequently if one of t l5enl . bundle" ; r 
obscure by the interposition of an absorbent medium in the course of u # , u o- 
corresponds with it. it becomes easy t„ estimate its intensity, ttbe ’ 0 7the^ 
ITT ° f ,be corresponding with the other half, the two par* 
have been rendered uniform in tint. , , -!*,*» ^ 
arriving ,lsht from a strong lamp may he ^ 
at the ■P|»rtttu* i this light should traverse a system of ab» . cipi J * 
S™* * rr r tlm lhe "^Transmitted may belong to one ot ^ 
,r. ! 1O0 f r i ‘ tlC with »hc solar spectrum. It is better, 
,ght 0 the spectrum, by receiving on a screen furnished a ,J b y i<;4 
vertical spectrum formed m the usual manner by rays rendered F ‘| ther ays*" 
lens (the principal lines of Fraunhofer should then be seen distinctly), 
