15 



II. — Eye-pieces with Adjustable Compensation. 



By H. Hartridge, M.A., M.D, F.RM.S., Fellow of King's 



College, Cambs. 



{Read December 18, 1918.) 

 One Diagbam, 



The importance of completely eliminating chromatic difference of 

 magnification has been dealt with in my investigations into the 

 resolving power of the eye {!)* for I found that the presence of 

 this defect had a markedly detrimental effect on definition at the 

 fovea centralis of the eye. 



Now, as is well known, m'icroscopic objectives vary greatly in 

 the amount of chromatic difference of magnification which they 

 exhibit. In some combinations the aberration is practically 

 absent, in others (particularly apochromatic objectives of high 

 power) its complete elimination is difficult, if not impossible, while 

 between the two extremes are found a large number which form 

 the semi- apochromatic class. 



There is thus found a considerable variation between the 

 amount of correction which different objectives require, and the 

 case is not properly met, therefore, by the provision of two sets of 

 eye-pieces only, namely, compensated and uncompensated; it is 

 necessary, in addition, that the amount of compensation may be 

 varied so that complete correction may be obtained in every case. 



The Holoscopic series of eye-pieces, which possess this property 

 of adjustable compensation, are found at the same time to suffer 

 from disadvantages, for not only is change in compensation accom- 

 panied by change in position of the lower focal plane, and therefore 

 change in focus of the objective, and change in tube length 

 (accompanied by the introduction of spherical aberration), but 

 also a change in the magnification is found to take place. Now, 

 although the new Holoscopic eye-pieces are better in this respect 

 than the old, still the disadvantages remain, but in a reduced degree. 

 Further, as the Holoscopic eye-piece is of the Huygens type the 

 Ptamsden circle is close to the upper surface of the eye lens. 

 This, in high powers particularly, causes inconvenience owing to 

 the closeness of the eye to the top of the eye-piece. 



A number of experiments have been made in order to obtain 

 an eye-piece which, while providing adjustable compensation, does 



* The italic figures within brackets refer to the Bibliography at end of the 

 paper. 



