54 PHOTO-MICROGRAPHS. 
low powers have greater penetration. If, therefore, 
we can secure satisfactory definition with a one- 
inch objective at a distance of eight or ten feet, 
the result will be better than that obtained by the 
use of a higher power at less distance. 
When the eye-piece is used in combination with 
the objective for the purpose of obtaining greater 
amplification, the screen cannot be removed to 
any great distance from the instrument, for the 
eye-miece magnifies any unperfections in the correction 
of the objective, and introduces additional unperfections 
of ws own, which interfere with the definition of 
the projected image. For this reason, the best 
results are obtained by removing the eye-piece. 
The limit beyond which no increase of amplifi- 
cation, by increasing the distance of the screen, 
can be obtained without a sacrifice of definition, 
differs for different objectives, and depends partly 
upon the perfection of the objective as an opti- 
cal instrument, and partly upon the fact that in 
ordinary objectives the corrections are made to 
give the best results at a definite point, namely, 
at the usual position of the eye-piece. 
A series of objectives made especially for pho- 
tography, and corrected to give the best possible 
definition at a distance of eight or ten feet from 
the screen, would possess some advantage over 
those not made for this particular work. 
Lenses provided with cover-correction admit of 
adjustment, within certain limits, for increased dis- 
tance ; but it is not easy to make this adjustment 
when the cover-correction is out of reach, and 
