On the Estimation of Ajperture. By Prof. E. Ahle. 21 



Calorific and Luminous Rays and the Limits of its Efficiency," f 

 another proposition pertaining to radiation in different media, viz. 

 that the iwwer of emission of a body — in regard to heat as well 

 as to light — is not the same in different media, but varies in the 

 ratio of the squares of the refractive indices, so that the ivhole 

 emitted light from any surface-element of a self-luminous body is 

 increased in the proportion of 1 : n"^ when this body is brought 

 from air into a denser medium of refractive index n. If a glowing 

 body at a constant temperature, such as a bar of iron, could be 

 immersed in a medium of 1'5 refractive index, in such a way 

 that the surface were in optical contact with the medium, and the 

 eye of an observer immersed likewise (the diameter of the pupil 

 being kept unaltered and the loss of refraction at the cornea com- 

 pensated for), the body would be seen brighter in all directions in 

 the proportion of 9 : 4 than it appeared in air. 



The principle of Clausius applies also to the diffused radia- 

 tion of non- self-luminous bodies, provided their internal structure 

 and surface are not changed by the surrounding media. An 

 object which fulfils this condition (without which of course there 

 could not be a constant illumination) — for instance, a pohshed 

 plate of porcelain glass — gives out by diffused reflection or by 

 diffused transmission a greater portion of the incident light, if the 

 radiation takes place into oil or in any other dense medium, than 

 when it takes place into air as can be shown by a simple 

 experiment, t 



The principle of this varying emission in different media is not 

 so far from a rational explanation as it may appear on a superficial 

 consideration. " Quantity of hght " is the energy of an undu- 

 latory motion. A " constant illumination," or equal intensity 

 of radiation, means equal ainpUtude and equal frequency of the 

 undulation at the radiating surface. These circumstances being 

 equal, the amount of undulatory energy which is transmitted by 

 the waves to any definite surface (for instance, to the whole 

 surrounding hemisphere) must depend on the density of the 

 propagating medium which is excited by the primary motion — 

 because the vis viva of every single wave of given amplitude is 

 greater in the proportion of this density. In fact, the stroke of a 

 bell or the human voice is found to give a louder sound in the dense 

 atmosphere at the level of the sea than in the rare air on high 

 mountains. According to the theory of Fresnel, the relation of the 



t " Ueber die Concentration von Warme- und Lichtstralilen," &c., Pogg. 

 Anualen d. Pliysik, cxxi. 1864. 



X The author has furnished to Mr. Crisp a little piece of apparatus for demon- 

 strating ad oculos the fact, that a thin polished plate of porcelain glass illumi- 

 nated from the back, throws, from a given area, an evidently greater quantity 

 of liglit into a block of crown glass (cemented on), than an equal area of tlie same 

 plate under exactly the same illumination throws into air. 



