The President's Address. By Prof. P. Martin Duncan. 181 



Dr. Gr. E. Blackham has made some very practical remarks on 

 the correction adjustment for homogeneous-immersion objectives,* 

 which have appeared in our Journal. He meets the reasons 

 for dispensing with an adjustment, viz. no risk of decentering, the 

 existence of a one best position in all objectives, the cost of the 

 adjustment, and the trouble. He very properly remarks that the 

 decentering need not take place if the optician does his duty to 

 the brass as well as to the glass, and that cost is quite out of the 

 question if the thing is possible and is required. And he observes 

 that although the shifting of the positions of the systems of lenses 

 is only an expedient, yet if it can be shown that it reaches the 

 desired end more certainly, speedily, and accurately than any other, 

 the objection to it must fall to the ground. Dr. Blackham qualifies 

 the term homogeneous immersion, stating that it is true as to the 

 idea but not in practice, for there are differences between the 

 refractive powers of the front lens of any objective and the medium, 

 and the refractive powers of different samples of crown glass are 

 not the same. In alluding to the method of correction he appears 

 to favour Mr, Wenham's mechanical process. He considers that 

 the small adjustments can be made with more ease, rapidity, and 

 accuracy by means of the screw-collar moving the back system of the 

 objective than by means of the draw-tube. 



Our Honorary Fellow Dr, Dippel considers all this, and whilst 

 admitting that the theory of correction is true, believes that most 

 of the arguments in favour of correcting combinations of lenses on 

 the homogeneous immersion are fallacious or of no value and 

 weight. He admits that with the correction-collar we are not so 

 strictly limited to an immersion fluid of a particular index of 

 refraction, and also that the correction-collar allows the same 

 objective to be used with a longer or shorter tube, whilst its absence 

 entails the employment of an objective within very narrow limits 

 of tube length. He would have an average correction made and 

 all screwed down hard and fast, believing that more errors will 

 ensue after meddling with the normal correction than occurred 

 before without any attempt at correction. 



In concluding these references to the correction of objectives 

 I cannot do better than quote Professor Abbe : — " Increase of 

 aperture is inseparable from a rapid increase of sensibility of the 

 objectives for slight deviations from the conditions of perfect correc- 

 tion. 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 portion of the pre- 

 paration 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, 

 * Proc. Amer. Soc. Micr., 1881, pp. 61-4. 



