1881.] 



MICROSCOPICAL JOURNAL. 



197 



of wide angular apertures that the 

 habitual use of deep eye-pieces to 

 obtain amplification has been ad- 

 vocated, and even now it is only for 

 " wide-angled and first-class object- 

 ives," as our correspondent states. 

 The explanation is simple enough, — 

 as the aperture is increased the ob- 

 jective admits more light, and the 

 use of deeper eye-pieces is thereby 

 rendered possible. It is not that any 

 advantage has been observed in the 

 use of deep eye-pieces and compara- 

 tively low-power objectives. We are 

 willing to admit that, within reason- 

 able bounds, the definition is as good 

 in one case as in the other. But theo- 

 retically this cannot be true, and prac- 

 tically there is greater fatigue to the 

 eye when deep oculars are used, and 

 it is the practical aspect of the ques- 

 tion that most interests us. A few 

 words, however, about the theory. It 

 has been clearly shown and already 

 stated in this Journal (p. 88), that 

 the imperfections of the olijective 

 are so much greater than those of the 

 ocular that the latter may be disre- 

 garded, and that it is useless to at- 

 tempt the correction of the former 

 by carefully corrected oculars. It fol- 

 lows from this that the use of dea^p 

 oculars magnifies the defects in the 

 objective-image, and for this reason 

 it is better to use shallow oculars and 

 well-made high-power objectives. But 

 lest any person should be led to a 

 wrong inference from this remark, we 

 should say that it is only in very ex- 

 ceptional cases that an objective of a 

 less focal length than the ^^ of an 

 inch is of any use, and even then it 

 should be of a moderate angular 

 aperture ; for extreme apertures a 

 focal length of yV of an inch is, in 

 our opinion, quite high enough for 

 any purpose. The practical side of the 

 (juestion is easily disposed of from 

 the testimony of experienced investi- 

 gators, and also from other consid- 

 erations. It seems that the fatigue of 

 the eye occasioned by the use of deep, 

 oculars would naturally result from 

 the necessarily more careful adjust- 



ment of the eye to the focal point, 

 and from the greater angular conver- 

 gence of the cone of rays which enters 

 the eye for a given amplification. 

 When a writer asserts that he can 

 distinctly observe certain points of 

 structure with a low-power, wide- 

 angled objective and a deep ocular, 

 we do not doubt the statement; but 

 probably the same details could be 

 continuously studied with less strain 

 to the eye, with a higher power ob- 

 jective and a lower ocular. As regards 

 the wide angular apertures under deep 

 eye-pieces, it should be remembered 

 that the greater the angle the less per- 

 fect the correction of all the rays from 

 the centre to the periphery of the 

 field, hence the greater the defects in 

 the image to be revealed by the deep 

 ocular. It can now be readily under- 

 stood why we would prefer to work 

 with a yVi'^ch of a given aperture up 

 to the limit of its resolving power, 

 with a low ocular, rather than with a 

 ^-inch of the same aperture, with a 

 correspondingly deep ocular. 



Since the above was written we 

 have received a letter from a gentle- 

 man whose name is well known to 

 the readers of this Journal, but as 

 the letter was not intended for publi- 

 cation, we suppress the writer's name, 

 and quote a passage bearing upon 

 this subject : — 



"I was much pleased with your 

 article on 'Objectives' in your last 

 number. Great claims were made 

 for the high angled, homogeneous- 

 immersion lenses, — that they gave bet- 

 ter definition, and allowed the use of 

 much higher power eye-pieces. Well, 

 I ordered the best ToUes could make, 

 and I have tried it to my satisfaction 

 on bacteria, and I know that it will 

 not bear deep eye-pieces, nor is there 

 any very perceptible improvement in 

 definition over a water-immersion of 

 moderate angle by the same maker. 

 The short working-distance I think 

 is a most decided objection." 



This is the testimony of a man who 

 is engaged 'in original investigations. 

 o 



