306 Transactions of the Society. 



aperture is prejudicial to the ease and convenience of microscopical 

 work in two essential respects. 



Istly, It necessitates a progressive reduction of the working 

 distance of the objective. Owing to the rapid increase of the 

 anterior aberration with increasing obliquity of the marginal rays 

 (particularly in the case of dry lenses), perfect correction of a 

 system cannot be obtained unless the layer of low refraction 

 between the object and the front lens (i. e. the working distance) 

 is reduced to a certain fraction of the focal length of the system, 

 which fraction is necessarily diminished in a rapid proportion as 

 the aperture becomes greater and greater. Whilst there is no 

 objection to retaining as working distance -^V of the focal length 

 for an aperture of 30^, if the aperture is 60° not more than -^jy 

 can be allowed, and with an aperture of 116^ really good correction 

 is not reconcilable with a working distance exceeding ^\y of the 

 focal length. It is therefore an obvious disadvantage to use 

 aperture angles of 60° and of 116'^, when the power which is 

 required or available can be obtained with 30^ and 60'^ respec- 

 tively. 



2ndly, Increase of aperture is inseparable from a rapid increase 

 of sensibility of the objectives for slight deviations from the con- 

 ditions of perfect correction. The state of correction of an objective 

 depends on the thickness of the refracting film between the radiant 

 and the front lens, represented by the cover-glass and that por- 

 tion of the preparation which is above the actual focus. This is 

 a variable element independent of the objective itself. In order 

 to avoid large aberrations which must result from the change of 

 that element, its variation must either be confined to narrow limits 

 or must be compensated for by a corresj)onding change in the 

 objective. Now there is a great difference in regard to this 

 requirement between the objectives of low and of wide aperture, 

 in particular with the dry system. An objective of a few degrees 

 is almost insensible, it may be focussed to the bottom of a trough 

 of water without any loss of performance. With SC^ differences 

 in the cover- glasses within the usual limits are still inappreciable, 

 and an object may be seen at the depth of a drop hanging on the 

 under surface of a cover-glass. With 60° a deviation of the cover- 

 glass from its standard thickness by not more than ■ 1 mm., or a 

 corresponding increase of the depth of the preparation above the 

 actual focus, will introduce perceptible aberrations and a visible loss 

 of definition if not compensated for. With an aperture exceeding 

 100° in a dry lens, the same result will arise from a change of 

 thickness of 0*02 mm, only. To preserve always the best cor- 

 rection in such a system would necessitate a change of the 

 correction-collar for almost every change of focus in the inspec- 



