364 Mr. W. Crookes on the Measurement of the 



dows side by side, a position will at last be found at which the two shadows 

 appear of equal strength. By measuring the distance of each light from 

 the screen and squaring it, the product will give the relative intensities of 

 the two sources of light. 



In practice this plan is not sufficiently accurate to be used except for 

 the roughest approximations ; and from time to time several ingenious 

 contrivances, all founded upon the same law, have been introduced by 

 scientific men by which a much greater accuracy is obtained; thus, in 

 Ritchie's photometer, the lights are reflected on to a piece of oiled paper 

 in a box, and their distances are varied until the two halves of the paper 

 are equally illuminated. In Bunsen's photometer, which is the one now 

 generally used, the lights shine on opposite sides of a disk of white paper, 

 part of which has been smeared with melted spermaceti to make it more 

 transparent. When illuminated by a front light, the greased portion of 

 the paper will look dark ; but if the observer goes to the other side of the 

 paper, the greased part looks the lighter. If, therefore, lights of unequal 

 intensity are placed on opposite sides of a piece of paper so prepared, a 

 difference will be observed ; but by moving one backwards or forwards, so 

 as to equalize the intensity, the whole surface of the paper will appear 

 uniformly illuminated on both sides. This photometer has been modified 

 by many observers. By some the disk of paper is moved, the lights re- 

 maining stationary ; by others the whole is enclosed in a box, and various 

 contrivances are adopted to increase the sensitiveness of the e}^e, and to 

 facilitate calculation : but in all these the sensitiveness is not greatly aug- 

 mented, as the eye cannot judge of very minute differences of illumination 

 approximating to equality. 



In 1833 Arago described a photometer in which the phenomena of 

 polarized light were employed. This instrument is fully described, with 

 drawings, in the tenth volume of the ' (Euvres completes de Francois 

 Arago but the description, although voluminous, is far from clear. The 

 principle of its construction is founded on the law of the square of the 

 cosines, according to which polarized rays pass from the ordinary to the 

 extraordinary image. The knowledge of this law, he says, will not only 

 prove theoretically important, but will further lead to the solution of a 

 great number of very important astronomical questions. Suppose, for ex- 

 ample, that it is wished to compare the luminous intensity of that portion 

 of the moon directly illuminated by the solar rays, with that of the part 

 which receives only light reflected from the earth, called the partie cendree. 

 Were the law in question known, the way to proceed would be as follows : — 

 After having polarized the moon's light, pass it through a doubly refracting 

 crystal, so disposed that the rays, not being able to bifurcate, may entirely 

 undergo ordinary refraction. A lens placed behind this crystal will there- 

 fore show but one image of our satellite ; but as the crystal, in rotating 

 on its axis, passes from its original position, the second image will appear, 

 and its intensity will go on augmenting. The movement of the crystal 



