Proceedings of the P eotographical Section. 1078 



the prisms, coincided -vritli lines which diverged from the observer, it 

 is evident that if we pLice two pictures of say three feet in diameter 

 at the distance of ten feet, that when viewed through the prisms, 

 thev will appear to be brought together at the point of convergence. 

 When tliese prisms are phiced on the point or in front of this field 

 lens or opera glass, the eifect is very good, though while the opera 

 glass magnifies the image, it also decreases the size of the field, 

 making it necessary to increase the distance from the picture ia 

 order to take in the whole of it. 



He also exhibited a view lens, constructed on the principle of the 

 Stanhope eye glass. The stop or diaphram is formed by the glass 

 being turned away at the center; the glass being solid there are 

 only two surfaces at which the light sufiers refraction, and as all the 

 light has to pass through the small central opening, the oblique rays 

 are corrected for color in the same manner as in the unachromatized 

 globe or ratio lens. They may be made achromatic in the usual 

 manner, or may be made by cementing together two plano-convex 

 lenses, one of the lenses having been roughened so as to prevent the 

 light* passing through anywhere but at the central opening. They 

 give a^correct figure the same as the globe or ratio lenses. Mr. Boyle 

 stated that the one shown had much too large an opening for its size 

 and focal length, and therefore answers only to illustrate the princi- 

 ple. They are so small and portable that a traveling operator may 

 carry half a dozen in his vest pocket, and they may be made so cheap 

 as to be sold, like chestnuts, by the quart or pint, or the difierent focal 

 lengths in papers, after the manner of. mixed candies. 



Mr. Boyle exhibited an instrument as an attachment for one of 

 his binocular telescopes, which changes it into a binocular micro- 

 scope for viewing objects at a distance of about twenty inches, and 

 is therefore useful for studying the operations of insects without dis- 

 turbing them at their labors. The same attachment will also answer 

 for viewing binocular pictures with the telescope, wliich gives a fine 

 magnified efiect. 



The section then adjourned to the second Tuesday in December. 



[Inst.] 68 



