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apocbroiuatic ©Djcctivcs. 



By J. W. GiFFORD. 



MUCH has of late been written about the new glass, and 

 while some have exaggerated its advantages, others 

 maintain that inasmuch as the secondary spectrum is 

 removed, some of the best rays have been taken away, and that 

 the image is therefore weaker than with ordinary achromatics. 



Messrs. Powell and Lealand, having recently constructed a 

 i/ioth apochromatic of numerical aperture 1*5 for the writer, who 

 has also, through the kindness of Mr. Curties, of Messrs. Baker, 

 Holborn, been able to compare it with several apochromatics by 

 Zeiss, it occurred to him that a short description of the glasses 

 and an account of the conclusions drawn might be of interest. 



The tests used were the Podura scale and the l^uberde Ba- 

 cillus^ the beaded appearance of the latter when spore-bearing 

 making an excellent test object. 



The Apochromatics tried were : — 



(i) i/io in. oil immersion, N.A. i'5. 



(2) 2'5 mm. water immersion, N.A. \'2^. 



(3) 6'o mm. dry immersion, N.A. 0-95. 



No. I by Powell and Lealand, Nos. 2 and 3 by Zeiss. 

 These were compared w'ith the following ordinary Achro- 

 matics : — 



(i) 1/25 in., N.A. 1-38 \ 



(2) 1/20 in., N.A. I "5 >oil immersions. 



(3) 1/12 in., N.A. 1-43 ) 



(4) 1/8 in., N.A. 1-26, water immersion. 



(5) 1/4 in., N.A. of 100° dry. 



Nos. I, 2, 3, and 4 by Powell and Lealand; No. 5 by Swift 

 and Son. 



The new compensating eye-pieces were used throughout, 

 those by Zeiss magnifying 4, 8, 12, and 18 diameters, those by 

 Powell and Lealand, 10 and 20. 



The stands used were Swift's Challenge, with dry achromatic 

 condenser, of aperture 180^, and Nelson-Curties's new Student's 

 stand, as made by Messrs. Baker. 



