The President's Address. By Prof. P. Martin Duncan. 157 



that by means of such mounting fluids, the capacity of stereoscopic 

 binoculars with the higher powers is considerably enhanced. True 

 stereoscopic effect, as we have seen, requires a depth of vision 

 not less than the thickness of the object under observation — a 

 depth which, as already shown, increases in direct proportion with 

 the increase in the refractive index {n) of the mounting fluid. 

 If one object is in air when n = 1*0, whilst another is in a 

 solution of phosphorus, where n = 2'l, the depth of vision will 

 be more than doubled. Objects, therefore, that by reason of their 

 thickness could only afibrd an unsatisfactory stereoscopic efiect in 

 air may be seen in lull relief when mounted in phosphorus. 



Here, again, the deductions of theory were remarkably verified 

 by the recent exhibition of Surirella gemma, under the binocular, 

 with a ^^3^-inch objective. 



Relative Value of Objectives with Large and Small Apertures. 

 (" All-round Vision "), 



I now come to a much-vexed question, that of the relative value, 

 practically, of objectives of large and small apertures, in regard to 

 which a great variety of opinions have been promulgated. 



The oldest of these views was that which made the preference 

 between the two kinds of objectives, depend upon whether they 

 were to be used for the " ordinary purposes of the biologist," or 

 for the examination of diatoms or other lined objects. The objection 

 to this view is, that it assumes the only function of a large aperture 

 to be its resolving power, a much too restricted notion, and one 

 which deprives the working biologist of a most essential aid to his 

 observations upon structure. 



A more modern view errs in the opposite direction, and insists 

 upon the universal superiority of large apertures, so that work 

 done with small apertures will " have to be done over again." 



There is again a third view, still more recently put forward, 

 which goes much further than the preceding, 'and according 

 to which it, is impossible that wide apertures can give correct 

 images. First on account of the unnatural '' all-round vision " which 

 it is contended is obtained with them, and secondly by reason of 

 their supposed inherent defect in defining power, in consequence of 

 the dissimilar images presented by the different parts of the 

 enlarged area of the objective, with a confused image as the general 

 resultant. 



The want of exactness in the first two suggestions will suffi- 

 ciently appear, when we have formulated the grounds upon which 

 large apertures are shown to be indispensable for all observations 

 upon minute structure for which high powers are necessary ; but it 

 will be desirable first to point out the erroneous interpretations upon 



