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The Ojytical Quality of Mr. Tolles" \ Objective. 



allude to his notice of the ^-inch objective of my manufacture, and 

 owned by Mr. F. Crisp, of London. 



Without quoting his declarations and conclusions about that 

 objective, I refer the reader to the accompanying diagram, Fig. 1, 

 as an illustration of the general case. I offer it as an example of the 

 conditions under which balsam-angle can be obtained greater than 



can possibly proceed from an air-angle capacity of the maximum 

 extent. That corresponding maximum balsam-angle is, as all con- 

 stantly agree, 82^ very nearly. This is elementary and plain — 

 assuming, however, that the material of front or of front-surface is 

 1 • 525 refractive index. 



But to discuss the other case, i.e. of balsam-angle exceeding 82°. 



4 



