*SLl'iineTiCT of Bigh-imver Definition. 315 



keenness of vision and its measures as applied to the microscope. 

 Microscopists can draw largely, therefore, on such recorded observa- 

 tions. It has already been observed that the smallest angle at 

 which parallel hnes can be numerically distinguished with a high 

 power of 1000 diameters is 183 seconds, or in round nimibers 200 

 seconds. Now there is a singular congruity between this critical 

 angle and that recj^uired for fairly dividing the most difficult double 

 stars. 



The celebrated Mr. Dawes, so renowned for observing powers, 

 the discoverer of Saturn's inner gauze ring like a crape veil, proposed 

 " a Polaris," which has a small companion of the ninth magnitude, 

 18" '6 distant from the pole star, as a general standard test, if the 

 telescope and eye of the observer are good, for a power of 80 diame- 

 ters. jN^ow, in the field of the eye lens, this double gives a critical 

 angle of 80 x 1 8"" 6, or 149" nearly. Considering the brilliance of the 

 pole star compared with the faintuess of the companion, it is indeed 

 a very severe standard test for the human eye. But I will venture 

 to relate a few more examples of critical angles of definition : — 



Diameters. Critical Angle. 



There is a curious similarity of these critical angles, at which 

 the double stars were resolved, to the angles at which Nobert's 

 lines were seen with various instruments and powers. 



* Brovrntng's silver-glass telescope, lO^th inch aperture. 



t These examples of definition are taken from ' Celestial Objects for Common 

 Telescopes :' Kev. T. W. Webb. 



[Professor Edwaeds' paper " On Diatomacese." — Owing to great 

 pressure on our pages the continuation of this article is unavoidably 

 postponed.] 



