Notes. 



295 



so fitted; 2nd, rackwork to the draw-tube in 1887; and 3rd, diagonal 

 rack work was fitted to the coarse adjustments in 1897. The figure is 

 taken from the instrument recently presented to this Society by the 

 late Dr. Whittle. 



OBJECT-GLASSES. 



"With regard to Powell's early object-glasses nothing has been 

 published ; and as most of tbem have been put together so that they 

 cannot be unscrewed, 1 have only been able to examine a few. The 

 following is a list of some glasses in my cabinet, with their aper- 

 tures and optical indices, and with a record of such observations as I 

 have been able to make. 



Date. 



1838 



» 



1840 



1818 



Ante\ 

 1857/ 



Remarks. 



A fine lens. 



Very good lens. 



Well corrected. 



Well corrected. 

 ^Double fronts. 

 j at the back. 



Three graduated diaphragms 



Triple front and buck, double middle, marked 



covered and uncovered (fig. 85). 

 Single front, double middle, triple back, collar 



graduated. 

 Triple front and back, double middle, collar 



graduated (fig. 85). 



Fig. 85. 



In 1846 we find a ] /4 with a triple 

 front and double middle and back. 



With regard to the single front for 

 medium powers, such as the 1 /4 of • 7 

 N.A., Powell did not seem to find it 

 satisfactory, because he afterwards went 

 back to the triple front and back and 

 double middle (fig. 85). 



Powell was the pioneer of very high 

 powers; as early as 1840 he made a 

 1/16, in 1860 a 1/25, and in 1864 a 

 1/50. In 1857 he claims an aperture 

 as great as 175° for his 1/1 6, in which 

 year our President, Mr. Shadbolt, says 

 " its performance is deserving of the 



highest praise." In the Paris Exhibition of 1855 a water-immersion 

 lens by Prof. Amici was exhibited, and in the London Exhibition of 

 1862 water-immersion lenses were exhibited by Messrs. Hartnack 

 and others, those oi Messrs. Hartnack being better than those of other 

 foreign exhibitors, but his 1 mm. focus immersion was stated to be 



