100 COKRESPONDENCE. 



system iu this sense at all. It is placed in front of an already perfect 

 object-glass, and tlius destroys the previous conditions of vision ; for 

 example, its property of seeing objects in air or in vacuo. Whether 

 united or kept in two parts, the structure will still be simply an 

 objective plus a hemisphere in front. 



It is to be observed that the results of this discussion are entirely 

 negative. The question as to what is the cause of the superiority 

 observed in immersion glasses has been answered only so far as con- 

 cerns what is not the cause. At first it had been assumed, by a curious 

 oversight in theory, that these glasses were exempt from the reduced 

 angle of dry glasses ; and that their indefinitely wider angle accounted 

 for the difference. That both are reduced is the point which it has 

 required so much discussion to establish. Now that such easy methods 

 have been invented for the measurements, anyone can verify them for 

 himself. For all really useful purposes a rough measm-ement is quite 

 sufficient. A few degrees either way would not be of the slightest 

 consequence, as these, even supposing them real and not apparent, 

 could have no effect in accounting for the difference of performance. 

 The real question remains — what is the cause of this observed differ- 

 ence, and how far on optical principles can we accomit for it. On 

 this question — a much more difficult one — I purpose offering the 

 results of my investigations in a future paper. 



Your obedient servant, 

 S. Leslie Brakey. 



The Use of the "Aquatic Nozzle." 



To the Editor of the ' Monthly Microscopical Journal.^ 



Pennsylvania Park, near Exeter, July 12, 1873. 



SiK, — I have long watched what is expressively called " the battle 

 of the glasses ;" but it appears to me that one very decided recom- 

 mendation of the new (immersion) mode of using the highest powers, 

 has not, as yet, been distinctly stated. 



It is as follows : — 



For many years past the makers have been working up the " angle " 

 of achromatic objectives higher and higher ; because the increased 

 angle was supposed to have certain advantages ; which may be the 

 case. It has, however, one very great f?/sadvantage, which is that as 

 the angle is zHcreased, the focal distance is decreased ; thus rendering 

 it impossible to focus an object, unless the covering glass is of 'that 

 exquisite thinness which the " preparers " are not always sufficiently 

 attentive to use. 



Hence the late Judge Tyrrell, the inventor of the first " finder," 

 and who was a very intimate friend of mine, when I told him, many 

 years ago, that I had a sixteenth to show him, expressed sm'prise, say- 

 ing, " A sixteenth ! I never yet saw one that was usable ! " 



I soon, however, convinced him he was " taking the thing by the 

 wrong handle," as the saying is ; and that he cnight to say he had never 

 yet seen objects jirepared with sufficiently thin glass to enable the six- 

 teenth to reach them. When that is the case, a sixteenth is just as 



