PHOTOMICROGRAPHY. 117 
makers great pains are taken to make the actinic and visual rays 
meet in the same point; but as microscopic lenses as a rule are 
constructed solely for giving the best definition to the human eye, 
a different formula is adopted for the curves of the glasses. For 
the sake of your junior members, some of whom may be imper- 
fectly acquainted with optics, you will, perhaps, grant me a short 
indulgence while I explain the principle of achromatism. [Dia- 
grams were thrown upon the screen illustrative of the construction 
of achromatic lenses and the over-correction of microscopic ob- 
jectives. | 
As it is generally accepted that the best definition is obtained in 
photomicrography without the use of the eyepiece, you will at once 
see the necessity for allowing readily for this discrepancy between 
the visual and actinic foci. In the instrument before you I have 
determined by experiment that, when a two-inch object-glass by 
Baker is employed, the screen being thirty-seven inches distant 
from the object-screen which has been visually focussed, the ob- 
jective must be withdrawn 35, inches, or the rod turned till the 
pointer previously placed at zero passes through 15° to the right 
(at half the focal distance 73° must be allowed). The chemical 
rays will then make a sharp picture on the film, whilst the outline 
appears blurred to ordinary vision. In the more delicate corrections 
required by the higher powers recourse may be had to extension of 
the bellows instead of altering the fine adjustment screw. The 
allowance required for each objective can readily be ascertained by 
placing an ordinary micrometer scale ruled to z$5ths and jo455ths 
inch on the stage of the microscope at an angle of 10° to 15°, so 
that each line has a varying focal point. On the screen is a photo- 
graph of such a scale taken at thirty-seven inches by the two-inch 
lens, the visual focus having been adjusted at the first line of the 
thousandths, and the best photographic definition coming out 
about the fifth or sixth line of the hundredths. The difference in 
the visual foci of these two lines was represented by 15° on the 
scale, which was readily found by viewing the scale through the 
eyepiece and swinging the index between the two focal points. 
Instead of the micrometer scale an object not pressed too flat— 
such as a fly which has delicate hairs lying in different planes—may 
be employed in a like manner. 
If a specially sharp picture be required of some difficult object, 
trial plates should be taken with determinate variations in the 
length of the bellows, either by cutting a dry plate into slips— 
which should be numbered and exposed separately—or a dia- 
phragm can be placed immediately behind the focussing-screen, 
having a revolving disc three inches in diameter fixed in the centre, 
a quadrant being cut out of the same to allow of successive ex- 
posures, the disc being revolved and the shutter lowered between 
