1882.] 



MICEOSCOPICAL JOURNAL. 



129 



so far as the aperture is concerned, 

 yet we have done so at the expense 

 of the visibiHty of the resultant im- 

 age, which has become fainter by the 

 nearer approximation to equality of 

 the refractive indices of the diatoma- 

 ceous silex and the balsam; the visi- 

 bility of minute structures being pro- 

 portional to the difference between 

 the refractive indices of the object 

 and the medium in which it is 



mounted. 

 ******* 



Continuing his researches on this 

 subject, and endeavoring to find the 

 best media with high refractive in- 

 dices, he has quite lately brought be- 

 fore the Society the utility of an 

 aqueous fluid capable of being given 

 the high refractive index of 1.68, viz., 

 a solution of biniodide of mercury 

 and iodide of potassium in distilled 

 water. This more manageable and 

 highly antiseptic medium appears 

 likely to turn out to be of great use in 

 the observation of many objects, 

 as it can be diluted till the index 

 of water is obtained. This is of ad- 

 vantage with such objects as muscu- 

 lar fibre, which are themselves of 

 high refractive power, so that fluids 

 of low refractive power must be made 

 use of to obtain the required differ- 

 ence for more perfect visibility. The 

 same communication also contains 

 what was much wanted, detailed 



practical directions for mounting. 

 * * * * * * * 



In addition to the increase in visi- 

 bility, there is also the fact that by 

 means of such mounting fluids, the 

 capacity of stereoscopic binoculars 

 with the higher powers is consider- 

 ably enhanced. True stereoscopic 

 effect, as we have seen, requires a 

 depth of vision not less than the 

 thickness of the object under obser- 

 vation — a depth which, as already 

 shown, increases in direct proportion 

 with the increase in the refractive in- 

 dex {n) of the mounting fluid. If 

 • one object is in the air when n = i.o, 

 whilst another is in a solution of phos- 

 phorus, where n = 2.1, the depth of 



vision will be more than doubled. Ob- 

 jects, therefore, that by reason of their 

 thickness could only afford an unsat- 

 isfactory stereoscopic effect in air, may 

 be seen in full relief when mounted 



in phosphorus. 

 ******* 



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 objec- 

 tives 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 bi- 

 ologist," or for the examination of 

 diatoms or other lined objects. The 

 objection to this view is, that it as- 

 sumes 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 obser- 

 vations upon structures. 



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 preced- 

 ing, and according to which it is im- 

 possible 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 sufficiently ap- 

 pear, when we have formulated the 

 grounds upon which large apertures 



