Eye-pieces with Adjustable ComiJensation. 19 



In the case of the x 16 eye-piece the Eamsden disc was found 

 to be approximately 4 mm. above the lens (being practically the 

 same distance as that in a X 10 Huygenian eye-piece). This eye- 

 piece, in spite of its high power, was therefore found very 

 comfortable in use. 



Summary. 



The eye-pieces described possess the property of adjustable 

 ■chromatic compensation which enables them to be used for all 

 <?lasses of microscopic objective. Unlike the Holoscopic type of 

 •eye-piece change in compensation is not accompanied by change in 

 focus, magnification or equivalent tube length. In all powers the 

 ■change in compensation is effected by the movement of an a-focal 

 •combination consisting of a crown and a flint glass lens of equal 

 and opposite power. 



The eye-pieces are found to be suitable for ordinary visual 

 ^^•ork, micrometry and projection. 



Addendum. 



Since my experiments were completed and the above brief 

 description written, I have found that the use of an a-focal com- 

 bination has been previously made by Professor Abbe {2). This 

 is described in his words as follows : — 



" I therefore added a correcting lens below the eye-piece, a 

 convex flint and concave crown lens cemented together, calculated 

 for neutralizing one another in respect to the middle rays of the 

 spectrum, but with exceeding chromatism of the flint. This lens 

 therefore performs as a plain parallel plate for the middle rays, as 

 a collective lens for the blue, and as a dispersive lens for the red. 

 Inserted at the end of the tube by a suitable adaptor at a short 

 •distance from the field lens of the ocular, it introduces, owinsr to its 

 position, no perceptible aberrations, neither chromatic nor spherical; 

 but it corrects the path of the coloured pencils outside the axis by 

 continuously increasing prismatic deflections. This device performs 

 quite satisfactorily with all eye-pieces except very low ones." 



The use of the a-focal combination has therefore been antici- 

 pated by Abbe, but not apparently in the precise form that I have 

 adopted — namely, as a movable element. In this respect my eye- 

 pieces still appear to be novel. 



Eeferences. 



1. Hartridge. — Journ. Physiol., 1918, p. 17-5. 



2. Abbe.— Roy. Micr. Soc, 1879, p. 822. 



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