Mr. ]]'eiiha)iCs Position Regarding High Balsam Angles. 319 



year 1854 I first pointed out the fact — not till then noticed — that the angle 

 of aperture of an object-glass, however great in air, became reduced to 

 below 82° in Canada balsam. In 1855, in the same Journal, I demon- 

 strated how wide apertures, or angles greater than 82°, could be obtained 

 in balsam by the adaptation of an additional front lens, used with an 

 intermedium of the same refractive power as glass, such as Canada 

 balsam. This was completely successful, and I have the same lens 

 (a 4- inch of 120°) still in my possession. It works well with water, but 

 better still with glycerin, or a highly refractive oil. 



In the early stage of the controversy, the fact of reduction from an air 

 aperture to 82<' in balsam, was disputed, but it is a mistake to suppose 

 that I have declared that no angle beyond 82° could be obtained with an 

 object mounted in balsam. I have looked over the pages of the late 

 Monthly Microscopical Journal from the year 1870 to its end, and 

 examined the passages referred to by Mr. Stephenson in his foot-notes, 

 and find that I have not made a statement so utterly inconsistent with 

 the fact ; I first pointed out the balsam deficiency, and suggested and 

 carried out the remedy. 



At an early period of the controversy, I wrote the following sentence 

 which I now quote from the Monthly Microscopical Journal for June, 

 1872, page 273. "Mr. Tolles has accepted the only condition (the four 

 system) under which the full aperture can be brought to bear on a 

 balsam mounted object. It is the tiny hemisphere. I am glad of his 

 announcement that he has succeeded in this, and should like to see the 

 same thing done in this country, particularly with large aperture glasses, 

 say higher than y% inch." 



Since the 5 inch of 1855, a number of others having a long working dis- 

 tance have received the addition of supplementary fronts. For large im- 

 mersion angles, this never fails to give good results, in accordance with the 

 original perfection of the object-glasses without the fronts. 



Two of these supplemented one-fifths were sent to the late Philadel- 

 phia exhibition, therefore any attempt to attribute to me a denial of the 

 principle with which I have been so long acquainted, either arises from 

 mere prejudice, or is taken as an assumed ground for adverse criticism. 



I readily admit that a great deal of unnecessary irritation has been dis- 

 played, which time has shown to be worse than useless, as it has led to 

 recrimination, careless, and inconsistent wording. What shall be con- 

 sidered a standard apertometer ? is the question which I now wish to be 

 settled, as I have not yet succeeded in applying any method of my 

 own to the measurement of aperture, with which I am satisfied. 



London, June 21st, 1879. 



F. H. WENHAM. 



