1 60 MEMORANDA. 



being concentrated and brought as it were into a small com- 

 pass the detection of them is easy and certain. 



In the next division, or the 2^ to 5, we shall find the 

 moderate-sized diatomes ; and lastly, in the " not in 5," we 

 get a mass of the remaining and smaller diatomes, all of 

 which small ones are themselves the more readily seen and 

 identified when separated from their larger brethren. 



I would venture to add, moreover, that I think the exami- 

 nation of these deposits for the various species is much faci- 

 litated, as the slides containing the to li sediment may be 

 examined with the inch objective ; the i inch will do to 

 examine the It to 2i, and 2i to 5 ; while the i inch need 

 not be used till we come to the " not in 5," whereas were 

 they all mixed the i inch would be required to examine the 

 whole. 



I should add, that what is poured off the large beaker A, 

 after it has stood the half-hour each time, may be flung away 

 and the sediment only transferred to the small beakers, as 

 from the large size of it there will rarely be more than 

 2 inches depth of water in it, and half-an-hour is ample time 

 to ensure every diatomaceous particle atom falling to the 

 bottom and being preserved and detected in one or the other 

 of the divisions. — F. Okeden, C.E. 



Aperture of Object-glasses. — As my friend Mr. Sollitt consi- 

 ders the principle that I have proposed, for measuring the 

 angle of aperture of object-glasses, is both complex and 

 erroneous, I may briefly remark, that he himself supports me, 

 with the most substantial evidence possible, of his own per- 

 fect conviction of its utility. Simply, because the very 

 method that he has proposed as a substitute, both in its 

 action and use, is absolutely identical with my own ; which 

 is, " to use the object-glass of the microscope as the objective 

 of a diminishing telescope." This I accomplished by placing 

 a biconvex lens over the eye-piece. 



The next point in question is, with respect to the angle of 

 aperture being reduced upon an object, immersed in balsam ; 

 wherein Mr. Sollitt is of opinion, that both the results of 

 Professor Robinson and myself are erroneous. 



The methods of measuring apertures under the latter con- 

 ditions must, of necessity, be altogether different, and form a 

 distinct branch : at present I know of no principle that will 

 serve the purpose so well as that proposed by Professor 

 Robinson. The experiments that Mr, Sollitt has brought 

 forward to prove that it is an error to suppose, that the angle 

 of aperture is reduced in balsam, amounts to no proof at all, 



