Ross's Four-Tenths Condenser. 59 



Oh. 47m. f again.— October 31, 21h. 13m. § p :— 221a.. 11m. 

 drawn ^nearer ; " flying clonds prevent estimations of the 

 distance." — 23h. 13m. "no longer visible." From snch very 

 limited and incomplete data it does not seem too much to say, 

 with all due respect to the illustrious observer, that no exact- 

 ness of period could be deduced. And nothing has since been 

 added to them. In an accompanying diagram the imaginary 

 spots are set nearer to or further from the edge of the ring, 

 to make the calculations correspond with the appearances;, 

 but he expressly states that his period applied to the exterior 

 ring A. Laplace had previously given about the same value, 

 but for the interior ring. 



(To be continued.') 



A near appulse of the moon to Saturn during the evening; 

 of Aug. 16 will be an interesting spectacle, the distance, at 

 Greenwich, being only 36'. 



BOSS'S FOUR-TENTHS CONDENSER. 



The subject of achromatic condensers for the microscope is 

 not yet exhausted, and although several forms of considerable 

 merit are now before the public, there is still room for experi- 

 ment and invention, both in the glasses employed and in the 

 nature of the stops. Mr. Ross has added to the number of 

 condensers one that especially deserves notice and commenda- 

 tion ; and, we think, after giving it a considerable trial, that it 

 will be regarded with great favour by practical microscopists. 



Before describing the new condenser, we must say a few 

 words on the different results which proceed from employing 

 combinations of different sizes and different degrees in angle 

 of aperture. Let the microscopist draw two lines converging 

 and making an angle of 100°, and then draw, at various dis- 



