660 SUMMARY OF CURRENT RESEARCHES RELATING TO 



" This correction, however, is not a complete one ; for, with the most 

 favourable distance between the two lenses, a not inconsiderable 

 remnant of the chromatic aberration still remains, while the spherical 

 aberration is already correspondingly overcorrected. The first is 

 noticeable by the blue edge bordering that side of the object which is 

 turned towards the centre when the object is placed towards the edge 

 of the field. The remnant of the spherical aberration causes the dis- 

 tortion and want of sharpness of definition at the edge of the field. 

 By increasing the distance between field-lens and eye-lens the blue 

 colour may indeed be made to disappear, but the spherical aberration 

 increases correspondingly, and the field is narrowed considerably. If, 

 on the contrary, the field-lens is brought closer to the eye-lens, the 

 spherical aberration is certainly diminished ; but, notwithstanding 

 this, the image at the edge of the field does not become any more 

 sharply defined, because the chromatic aberration has increased in 

 equal ratio. 



" One advantage, however, is gained by approaching the field-lens 

 closer to the eye, namely, a considerable widening of the field. 



" If, under these circumstances, the aberrations of the eye-lens are 

 corrected by suitably composing the same of flint- and crown-glass, we 

 have an eye-piece which, with all the advantages of the Huyghenian 

 eye-piece, surpasses the latter by having a larger field. 



" These facts form the basis of the construction of the Kellner 

 orthoscopic eye-pieces. Kellner brought the field-lens into the focus 

 of the eye-lens, made the latter achromatic, and chose such curvatures 

 as to remove also the spherical aberration, and showing a flat field, 

 for which latter purpose he also transformed the plane convex field- 

 lens into a double convex one. 



" The simultaneous accomplishment of all these results was favoured 

 by the circumstance that in approaching in a Huyghenian eye-piece 

 the field-lens to the eye-lens, the spherical aberration diminishes more 

 rapidly than the chromatic. The preponderance of the latter over the 

 former in the Huyghenian eye-piece must therefore admit of being 

 equalized at a certain point, or rather must accommodate itself at this 

 point to a similar disproportion in the achromatic eye-lens. This 

 point, however, is, as in the Kellner eye-piece, almost exactly the 

 focus of the eye-lens. 



" A further approach of the field-lens to the eye-lens (bringing the 

 latter within the focus of the former) again gives the preponderance 

 to the chromatic aberration, and an equalization by an achromatic 

 double lens becomes impossible imder the circumstances. 



"If, however, such further approach should be possible without 

 such or other disadvantages, it would be very desirable, not only on 

 account of the enlargement of the field which it would cause, but also 

 the circumstance that when the field-lens is in the exact focus of the 

 eye-lens, every fine particle of dust on the former is clearly visible 

 and sharply defined, greatly interfering with the observation. 



" These facts and considerations caused me to reflect whether a 

 triple eye-lens (consisting of two positive crown-glass lenses, and one 

 negative flint-glass lens) instead of a double lens would not better 



