PHOTO-MICROGRAPHY. 77 
Perhaps it may be useful to some of you to give the various 
corrections found to be necessary with several objectives in order 
to obtain the sharpest definition, and I may here advise all who 
wish to practise the art, to spare no pains in getting the sharpest 
definition possible, as having once obtained it, the correction will 
be constant for that objective. The microscope stand which I use 
for this work is one of Browning’s, the fine adjustment withdraws 
the objective from the stage jth of an inch for each complete 
revolution of the milled head when the screw is in the centre of 
its traverse. This milled head is divided into ro principal divi- 
sions, each of which is again separated into four. 
DESIGNATION. MAKER. ANGLE. TURNS, INCHES. 
amine lita ed oli clot Dancer 9° 2 sho 
DEP SER is onc Pe a 14° I on 
Bee G Bates Tie ed Browning ae 4 aa 
EMAAR Seco achat Gees e 60° in ae 
aT Rs Pen aaee ds Sk “5 85° i aie 
eh i maaees Ase es ‘ 140° ) ° 
RRM Fe catia eo Dancer 170° fo) fo) 
It Hee ‘been stated by several writers on this subject that mono- 
chromatic light brings both chemical and visual foci into the same 
plane, and therefore no correction is required. I cannot under- 
stand how any writer can have fallen into this error, for error it is, 
and which a single experiment would have set right. Of course 
the chromatic aberration is disposed of, but we have introduced 
a new element into our calculations ; a lens which is spherically 
corrected for white light is not so for monochromatic, and _ there- 
fore the picture is blurred with spherical aberration. 
There now remains but to describe the method of illumination: 
well, there is the lamp, either an ordinary thirty hole argand gas 
burner, the rays being manipulated with the bull’s-eye condenser, 
or the ordinary paraffin microscope lamp of no special pattern. 
The substage achromatic condenser is only used for powers of 
greater amplification than the half-inch, or where the camera is 
drawn out to a great extent. I have used several sources of 
illumination, sunlight, the electric light, the oxy-hydrogen lime 
light, the magnesium light, as well as a paraffin lamp and gas, but 
for general work the two last are preferable, seeing that they give 
nearly all the light required for medium and low powers, and if a 
very intense illumination is requisite, it can easily be obtained 
from gas or oil in such an apparatus as the Sciopticon. Dr. Carl 
Seiler, in a communication to the American Journal of Microscopy, 
p- 159, 1879, upon this subject, seems to think that wet plates are 
preferable to dry, and adduces arguments in support of his views. 
I cannot agree with him, however; dry plates are so convenient, 
and with good manipulation are capable of producing excellent 
