ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 345 



of the plate A are close to one another and are of equal size, the same 

 quantity of light (very approximately) will be incident upon them ; 

 and as the plate A is of porcelain and polished, neither the internal 

 constitution nor the surface undergoes any change by the cement 

 connecting it with the glass block. Thus the two fields /, and f c^ are 

 under equal conditions, with the difi'erence, however, that /i emits 

 rays into balsam and crown glass, and /2 into air. 



The plate B, which is only laid on the glass block, receives all the 

 light which has been emitted into balsam from /j upon the opposite 

 surface gi of the glass block, and all which has been emitted from 

 f^ into air and upon the field g^. As the former field is brighter than 

 the latter, the quantity of light emitted in the one case must neces- 

 sarily be greater than in the other, and the more so since the absorp- 

 tion of light is certainly greater in the glass block. 



Owing to the identity of all geometrical conditions, it is obvious 

 that every point P^ of the surface /i throws upon the field g^ the 

 same solid cone of rays as the corresponding point Pg of f^ throws on 

 g.2, and that every point Qj^ of the field g^ receives an equal cone to 

 the corresponding point Q2 of the field g^. Consequently, the inten- 

 sity of emission in balsam must be greater than the intensity of 

 emission in air, whilst the illumination of the object and all other 

 circumstances are the same. A similar difference of emission must 

 take place in all directions round the points Pi and P2 ; and thus the 

 whole of the emission in balsam (i. e. the quantity of light conveyed 

 from every unit of surface within the whole hemisphere) is seen to 

 be greater than the whole in air, a given fixed illumination being 

 supposed.f 



The idea of an unequal photometrical equivalent of equal 

 pencils in different media may be developed by some simple expe- 

 riments, which also show how absurd is the notion that there is a 

 loss of aperture (or of light apart from mere partial reflection) when 

 a dry objective is applied to a balsam-mounted object. 



(1) Any object in air (Fig. 86) will send into the pupil a pencil 

 of a given angle u* from every point, and is seen (with any given fixed 

 illumination) with a certain brightness.^ 



t This does not, of course, imply the assertion that the whole of emission from 

 such a porcelain plate, or any other object with diffused radiation, would increase 

 cordinually as the refractive index of the surrounding medium is increased, which 

 would lead to the absurd inference that such an object could give out more light 

 than is incident upon it ! If the index of the medium should exceed the refrac- 

 tive index of the object, the radiation (whether of reflected or transmitted light) no 

 longer embraces the whole hemisphere, but breaks off at a given obliquity. This 

 is shown from the consideration that the emission of light is not confined to the 

 mathematical surface of the object, but arises from a layer of finite (though very 

 small) depth. Thus, the total amount of reflected or transmitted rays from an 

 object reaches a maximum with a given density of the surrounding medium, 

 thougli the emission within any narrow cone (as long as there is emission still) 

 will always increase with increasing density. 



X If the distance is increased and the angle u* diminished, the area of the 

 retina which is affected by the rays from a given area of the object is of course 

 diminished as the square of the distance, i. e. in the same ratio as the number 

 of the rays from every point is diminished. Every sq. mm. of the retina receives, 

 therefore, the same quantity of light at all distances, and the object, therefore, 

 always appears of the same hrightness. 



