ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 315 



One of the more important practical mistakes which arises from this 

 error may be seen in the case of the higher-angled objectives, to 

 which we refer hereafter.* 



From the erroneous assumption of the identity of radiation in 

 different media spring innumerable fallacies. When in the old theory 

 of the solar system the earth was believed to be the centre, with the 

 sun and planets revolving round it, all kinds of complicated pheno- 

 mena appeared to exist, necessitating equally complicated explana- 

 tions, which gave rise to inextricable confusion. As soon, however, 

 as the sun was made the centre, the complications existed no longer 

 and required no explanation. In exactly the same way the angular 

 aperturists are misled. They not only miss the whole point of the 

 superiority of immersion objectives, viz. their larger apertures, but in 

 consequence of missing it, and yet being obliged to recognize that 

 they have advantages, they are induced to propound the greatest 

 absurdities. 



For instance, some will say that the advantage of immersion 

 objectives consists almost entirely in their increased working distance 

 and in their dispensing with the necessity for a correction collar. 



Some, discovering that there is a vast increase of light with 

 immersion objectives, explain their value to rest exclusively on the 

 reduced amount of reflection at the plane surface of the objective, which 

 in the case of the dry lens is said to reflect back a great part of the 

 light. These, through the erroneous assumption with which they 

 started, have been diverted from working out the calculation, which 

 shows the loss of light to be only about 10 to 12 per cent.f 



Others deny that immersion objectives can have any substantial 

 advantage as regards aperture over dry objectives used on objects 

 which are in air, as the latter radiate light up to the whole 180°, of 

 which the dry lens can take up, say 170°, so that there is little or no 

 room left for any improvement on the part of the immersion objec- 

 tives, which are therefore supposed to show superiority over dry lenses 

 only in the case of the latter being used on balsam-mounted objects f 

 This is the view which is most frequently — we may say invariably 

 — propounded, as it appears to be the most self-evident. 



Thus, if Fig, 61 represents a pencil of 170°, radiating from an 

 object in air, a dry objective of maximum aperture may take it up. 

 If, however, the object is mounted in balsam, it is supposed to be so 



Fig. 61. 



J70°INAIR 



" environed " that by far the larger part of the original pencil is 

 reflected back from the cover-glass (see Fig. 62) ; the dry lens, it is 

 said, is as ready as ever to take up the original 170°, but cannot now 

 get that pencil as it could before, when the object was in air. All it 



♦ See infra, II. " Angular- Aperture Fallacies," No. 5, "Fallacies in Practical 

 Construction." 



t See infra. III., No. 1, " Difference of Radiation in the same Medium." 



