58 
POPULAR SCIENCE REVIEW. 
Jens than the yellow and red rays which make up the visual 
image seen upon the screen. 
This chromatic aberration must, of course, he corrected in the 
best objectives by the proper combination of glasses with varying 
refractive powers; but, unfortunately for photographers, they are, 
invariably in the lower powers, overcorrected to compensate for 
some undercorrection in the eye-pieces, so that, instead of finding 
the actinic focus at a point nearer to the lens than that of the 
red and yellow rays, it is actually thrown farther off, and lies at 
some point beyond the image which is focussed on the screen. 
Various plans have been suggested for readjusting this differ- 
ence, but the simplest and least costly will be presently de- 
scribed under the head of “ turning out,” by which the distance 
between the visual and actinic focus is accurately ascertained 
for each separate lens. 
Coarse Adjustment , — When there are two adjustments attached 
to the microscope, it will be found advisable to reserve the coarser 
exclusively for focussing the object, and to leave the fine screw 
for the equally delicate operation of compensating for the differ- 
ence of visual and actinic foci. Of course, in a long camera both 
screws are far beyond the reach of the hand when focussing ; 
but a very simple contrivance will enable the operator to obtain 
the greatest possible delicacy. A stout brass wire (K) is bent 
round at one end so as to clip firmly the milled head of the 
coarse adjustment (I) ; a lever is thus obtained, the distal end 
of which will move through an appreciable space for the 
minutest variation of focus. To this lever, a long wooden rod 
(L) , reaching to the focussing screen, is attached by a loop of 
wire, with a small piece of perforated cork below to support and 
keep it in place. 
Fine Adjustment — The fine adjustment will be found of great 
service in recording the differences of actinic and visual foci, 
especially in the higher powers when minute variations are met 
with. To mark these with certainty, a small dial plate of card 
(M) is attached to the body of the instrument, on which a fine 
wire index bent to clip the milled screw-head of the fine adjust- 
ment, and traverse with it, wfill measure the slightest alteration. 
Condensers . — Passing backwards in order, the next portion 
of the instrument which must be noticed is the condenser. For 
some purposes we have found an ordinary bull’s-eye lens of 
some 3-inch focus answer very well, but for the higher powers 
a greater condensation of light is required. Eeade’s hemispheri- 
cal drum is a useful form of glass ; by it the light is quickly 
centred and adjusted, whilst its very complete system of stops, 
throwing rays of light in any given direction, bring out the 
markings of Diatomacese and other test objects with the greatest 
facility. The ordinary achromatic condenser sold with most 
