632 General Notes. [ October, 
rected by suitably composing the same of flint and crown glass, we have 
an eye-piece which, with all the advantages of the Hughenian eye-piece, 
surpasses the latter by having a larger field. 
These facts form the basis of the construction of the Kellner ortho- 
scopic 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 the spherical aberration and show a flat field, for which latter 
purpose he also transformed the plano-convex field-lens into a double-con- 
vex one. 
The simultaneous accomplishment of all these results was favored by 
the circumstance that in approaching in a Hughenian 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 Hughenian 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 lat- 
ter within the focus of the former), again gives the preponderance to the 
chromatic aberration, and an equalization by an achromatic double lens 
becomes impossible under 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 on account of 
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 trip 
eye-lens (consisting of two positive crown-glass lenses and one negative 
flint-glass) instead of a double lens would not better answer the condi- 
tions, and I have in consequence succeeded in forming such a lens which 
answers the purpose in a very high degree. i 
My new “ periscopic eye-piece” consists of a triple eye-lens, a ee 
eonvex field-lens, the latter being situated within the focal distance of t 
former, and a diaphragm located in the focus of the equivalents of 
lenses. sal 
The field of the new eye-piece is considerably larger and flatter 
that of Kellner’s, and the image is sharply defined to the extreme get 
As the focus of this eye-piece lies behind the field-lens (the same ” 
Ramsden’s eye-piece), it is particularly suitable for micrometers, ajk 
cially as the division is distinctly and in correct proportion visible to 
extreme edge, which is notably not the case with Ramsden’s 7 iat ws 
A micrometer division placed in the focus of this eye-pieo? |” 
moreover, very perspicuously the high degree of the co jon of the = 
errations, while the image transmitted by an objective can be no 
ple 
