252 
NOTES AND MEMORANDA. 
correction corresponding to tlie thickness of the covering glass, while 
the chromatic aberration is scarcely aifected by it. In the adjustment 
for cover thickness at present, however, the relations of the aberra- 
tions are exactly the reverse, for by moving the front lens away from 
the other lenses of the system, the object-glass will become more 
chromatically than spherically corrected. An object-glass, therefore, 
v/hich has been properly corrected for medium cover thickness, for 
instance, can by means of this adjustment be adjusted so far only as 
to correct either the spherical aberration absolutely and leave the 
chromatic aberration under-corrected, or in the most favourable case, 
under-correct the chromatic aberration, and over-correct the spherical 
aberration in such a way as to leave both remnants of equal value. 
Besides this, the distortions spherical as well as chromatic are only at 
a minimum when the lenses are at a certain relative position to each 
other ; and as in objectives of high angular aperture these defects can 
anyhow not be quite removed, the extreme position will so increase 
them as to make not only the distortion and colour appearance at the 
margin of the field unpleasantly apparent, but to interfere seriously 
with the definition. 
Furthermore, the changing of the relative distances of the lenses 
composing the object-glass seriously disturbs the focal relation of 
these lenses, causing a change in the mag- 
nifying powers of the magnifying glass 
which interferes seriously with micro- 
metric measurements. 
To obviate all the above-mentioned 
difficulties, Mr. Gundlach has invented a 
new cover adjustment for object-glasses, 
in which he places before the front lens 
of the objective a transparent disk with 
parallel sides, capable of being moved 
by some mechanical means closer to or 
farther away from the front lens. The 
general mechanical construction or ar- 
rangement of the objective is shown in 
the accompanying woodcut from the 
Patent Office Report. In the space inter- 
vening between this disk and the front 
lens he places a transparent fluid of a 
refracting power equal to, or nearly equal to that of glass, affording 
thus, by increasing or diminishing the distance between the parallel 
disk in question and the front lens, and increasing or diminishing 
thereby the thickness of the layer of refracting fluid between them, 
a direct compensation for the increased or decreased glass cover of 
the object, and consequently of the aberrations, without disturbing 
the focal relations of the lenses of the object-glass, thus avoiding the 
increased distortions and the disturbance of the magnifying powers 
of the objective consequent upon and inseparable from the disturb- 
ance of the focal relations of the lenses composing the system of the 
object-glass. 
