136 CORRESPONDENCE. 



propose (probably in your Marcb number, if you will publisTi the 

 proposed article) to set forth the applicability of the illustration to 

 all immersion objectives. My proposition 'is that the balsam-mounted 

 object can, in the case of the immersion objective, be viewed with the 

 advantage of an angular pencil actually transmitted through the object 

 of approximately 170°, more or less. But it should be particvLlarJy 

 noted that this is not avowed for water as the medium instead of air 

 above the slide-cover, but instead of air a medium approaching closely 

 the refractive power of glass. Between immersion and dry objectives 

 this is the remarkable difference, with the latter only82^(-|-) can 

 be obtained. This Mr. Wenham long since showed to be the case, 

 and I would humbly remark that I was aware of this and of Dr. 

 Eobinson's demonstration before in " simple honesty " giving my first 

 illustration in your Journal for July last, and to which Mr. Wenham 

 so stoutly took exception. 



Eespectfully yours, 



EOBEET B. TOLLES. 



P.S. — Allow me to state as a fact of interest to microscopists 

 that the resolution of Amphipleura pellucida is not (or any longer) a 

 difficult achievement. With a true yo"^^- ^^ only 90°, even less than 

 75°, I have repeatedly and plainly lined the valve, using sunlight and 

 hlue cell, the specimen of A. pellucida being one kindly supplied to 

 me as a proper test specimen by Dr. J. J. Woodward, and received by 

 him from London. This performance was not dependent on any 

 special merit in the objective — any good immersion y^^h would do 

 the same thing. To accomplish this I used a narrow angled 1 in., 10° 

 swinging under the stage of the microscojie as a condenser. This 

 1-in. was placed at the incidence (obliquity to the axis of Mic.) 

 indicated above, viz. 45° or less. The focus for parallel (sun's) rays 

 a little outside of the object. 



E. B. T. 



On Positive and Negative Aberration. 

 By Dr. Eoyston-Pigott. 



To the Editor of the ^Monthly Microscopical Journal.^ 



Mr. Editor, 2, Lansdowne Orescent, Feh. 7, 1872. 



Sir, — As my paper printed in the Transactions of the Eoyal 

 Society has been honoured by criticisms by two of your contributors, 

 professing to point out errors of a mathematical nature, undetected 

 by the accomj)lished mathematicians who scrutinized it, you will, I 

 trust, permit me to observe that in a standard optical work the fol- 

 lowing passages occur, in which some of our Fellows may perhaps be 

 interested : — 



" We suppose in these pages light to proceed from right to left, 

 and positive lines consequently to be measured from left to right.* 



In the case of a converging lens Dr. Parkinson shows, as also 

 * Dr. Parkinson's, F.E.S., 'Optics,' page 13, 2nd Edition. 



