438 NOTES. 



OIL-IMMERSION DARK-GROUND ILLUMINATOR. 



N.A. 1*45, Equivalent Focus 69 mm. 



'This dark-ground illuminator, after Stephenson, is of a very 

 simple construction, and in use is most efficient. Its equivalent 

 focus is 6*9 mm., which enables it to yield a large image of the 

 source of light. Its N.A. is full 1'45, which is greater than 

 any dark-ground illuminator now made. Kadius, 18"5 mm. ; this 



(Figure full size.) 



surface to be silvered. Flattop, 8'06 mm. in diameter. Diameter 

 -at bottom, 21*5 mm. Thickness, 3*0 mm. Thickness of com- 

 plete plano-convex lens before top is cut off, 3'44 mm. Diameter 

 of stop, 11 '5 mm. It is most important that the thickness should 

 be worked exactly true to the formula. 



Glass: Chance dense flint, n = r6225 v = 36*0. 



EDWArD M. Nelsox. 



