ZOOLOGY AXD BOTAXY, MICROSCOPY, ETC. 



341 



(3) A dry objective of 180^ angle, worJcing on dry objects, is assumed 

 to represent the maxiinura of microscopical perfection as regards aper- 

 ture icJiich can never tlierefore be surpassed by an immersion objective. 



Will any one seriously maintain that a dispute on tliis point is 

 only one of nomenclature — that it is only a question of nomen- 

 clature to contend that immersion objectives present only incidental 

 and comimratively unimportant advantages over dry objectives, 

 except in the single case of the latter being used on balsam-mounted 

 objects, and to deny to them the quality which raises them, in regard 

 to aperture, above anything to which the most perfect dry objective 

 can ever attain ? a quality, moreover, which is not a merely abstract 

 optical consideration, but is in fact an essentially practical one in 

 regard to the performance of the Microscope upon the most minute 

 objects. 



Fig. 78. 



III. PiLotometrical Q,usstions connected with. Apertiu-e. 



(1) Difference of Radiation in the same Medium, — The 

 Lambert law shows that the quantity of light emitted by any in- 

 finitesimal surface-element (or "bright point ") varies with the obliquity 

 of the direction of emission, being greater in a perpendicular than 

 in an oblique direction. The rays are less intense 

 in proportion as they are more inclined to the 

 surface which emits them. 



If a (Fig. 78) is a radiant element emitting 

 light within a small cone u (of angle q) in a per- 

 pendicular direction, and also within an equal cone 

 u' in an oblique direction, the angle of obliquity 

 between the two being lo, then the quantity of light 

 emitted by the element a in the oblique direction, 

 and contained within the cone u' , is less than that 

 which it emits in the perpendicular direction and contained within 

 the cone u, though the cones are of equal extent (g). The relative 

 quantities of light in the two cones are as 3 : g cos ic ; so that a 

 pencil varies according as it is taken close to or removed from the 

 perpendicular. 



Owing to the different emission in different directions, therefore, 

 the quantities of light emitted by one and the 

 same element in one and the same medium, 

 by cones of different angle {10 and 10' , Fig. 79) 

 are not in the ratio of the solid cones, as 

 would be the case with equal emission in all 

 directions, but in the ratios of the squares of 

 the sines ; so that the squares of the sines of 

 the semi-angles constitute the true measure 

 of any solid pencil of rays. 



The simplest experimental proof of the imequal emission in 

 different directions will be found in the fact that the sun, or the 

 moon, or any similar bright spherical object, with so-called uniform 



