ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 347 



passing from (87) to (88), so great that at any rate it cannot be 

 accounted for by the loss of light from one reflecting surface more. 



The experiments must of course be made with illumination from 

 helow in order to secure equal illumination in the three cases, and 

 with objects which are not altered by immersion in balsam or oil. 



More striking still is the result of the following consideration. 

 Suppose an exact hemisphere of glass {n = 1 • 5) and 

 an object close to its centre and under the conditions Fig. 89. 



of (87) (Fig. 89). 



The object adhering to the glass is seen by the 

 naked eye or by a Microscope at the same plane at 

 which it actually is, and with the same brightness 

 as when in air (see Fig. 86) (the slight loss by 

 absorption and reflection not considered). It is, 

 however, amplified in the ratio of 1 : n. 



Apart from any consideration of the cause of this amplification, 

 the question necessarily arises, how is it possible that equal pencils 

 (i. e. of the same angle), the one in air (86) and the other in glass (89), 

 give the same brightness while in air (86) every square millimetre of 

 the object continues to be the same square millimetre, and in (89) 

 every square millimetre of the object is enlarged to n"^ X sq. mm. The 

 total quantity of light which is obtained from every square millimetre 

 of the object is obviously n'^ times more in (89) than in (86). There 

 can be no other answer than this : pencils of equal angle, the one 

 emitted in air and the other in glass, are different -p q^ 



things physically, though they are equal geometrically 

 — the pencil in glass contains n^ x the light of the 

 pencil in air. This conclusion is shown to be correct 

 by comparing (89) with Fig. 90, where the object is 

 in air but close to the plane surface (as in 88). Here 

 the pencil from the object, in order to yield the same 

 emergent pencil u*, must have an angle u in aii', which 

 is greater than u in (89) according to the law of refraction. Never- 

 theless this greater cone of the emitted rays brings out the same 

 brightness of image as in (89) under the same amplification. 



IV. Microscopical Vision and the Delineating Power of 

 Objectives. 



The consideration of aperture involves two distinct questions : 

 First, Can immersion objectives have any and what excess of aperture 

 over the maximum attainable by dry objectives ? and secondly, What 

 is the function of increased aperture ? 



Up to this point we have been occupied exclusively with the first 

 question (to which, in accordance with the practice hitherto in vogue, 

 we have given priority) the angular aperture theory insisting that a dry 

 objective of 180° air -angle represents the maximum aperture possible. 

 The mistake of this view and the establishment of the true view may, 

 as it has been seen, be demonstrated upon those ordinary dioptrical 



