Page:Elementary Text-book of Physics (Anthony, 1897).djvu/461

§ 364] the energy on unit area of $$ab,$$ we have $$I' = I \cos \theta;$$ or, the energy of the radiations falling on a given surface is proportional to the cosine of the angle made by the surface and the plane normal to the direction of the rays.

364. Photometry.—The object of photometry is to compare the luminous effects of radiations. It is not supposed that the radiations which fall on the retina are totally absorbed by the nerves that impart the sensation of light. The luminous effects, therefore, depend on the susceptibility of these nerves, and can only be compared, at least when different wave lengths are concerned, by means of the eye itself. The photometric comparison of two luminous sources is effected by so placing them that the illuminations produced by them respectively, upon two surfaces conveniently placed for observation, appear to the eye to be equal. If $$E$$ and $$E'$$ represent the intensities of the sources, $$I$$ and $$I'$$ the intensities of the illuminations produced by them on surfaces at distances $$r$$ and $$r',$$ the ratio between these intensities, as was seen in the last section, is and when $$I$$ and $$I'$$ are equal, $$Er'^2 = E' r^2,$$ or  That is when two luminous sources are so placed as to produce equal illuminations on a surface, their intensities are as the squares of their distances from the illuminated surfaces.

In Bunsen's photometer the sources to be compared are placed on the opposite sides of a paper screen, a portion of which has been rendered translucent by oil or paraffine. When this screen is illuminated upon one side only, the translucent portion appears darker on that side, and lighter on the other side, than the opaque portion. When placed between two luminous sources, both sides