After the lamp is lighted and all adjustments made, the apparatus may with advantage be allowed to rest for a short time, for the heat concentrated on the lenses, etc., is likely to cause slight expansion. The object can be focussed while the camera bellows is but extended one half. It can afterwards be extended until the image on the screen is of the desired size, and the fine adjustment brought into requisition as a finishing touch. The screen is now folded back, and the dark slide charged with its gelatine plate or plates is slipped into position. A blank card is placed against the sub-stage to shut off the light, the dark slide shutter is drawn, and now all is ready for taking the photograph. All that remains to be done is to remove the blank card for as long a time as may be judged necessary for exposure.

Upon this subject of exposure we can say but little, for it is governed, as in the case of an ordinary photograph, by a great many circumstances. One advantage however is possessed by the Microscopic worker, and that is that his source of light is a constant one. He is not dependent upon fickle daylight. (It may be mentioned here, that daylight or any other light except that proceeding from the lamp, must be rigorously excluded during all operations. But of course the red lamp may be kept burning with advantage, both as an illumination for the room, and for the necessary chemical work). But he will soon find out that length of exposure is governed in great measure by the nature of the object photographed. If the mounting medium (generally Canada balsam) be of a yellow tinge, this alone will necessitate greatly prolonged exposure. Then again, the object may be purposely stained with some nonactinic dye such as aniline brown. The best results will be obtained if the worker is clever enough to prepare his own slides. It would take us too far from our subject to go into this matter of mounting. But information upon the subject can be readily obtained from the various excellent books upon the Microscope now published. Of these we may mention Wood's "Common Objects for the Microscope," Davies "On Mounting Microscopic Objects," and the large treatises of Carpenter and Beale,

Weights And Measures

Photographic formulae are compounded by Apothecaries' Weight. Dry chemicals are preferably weighed out in scales with glass pans. Liquid chemicals are measured in a graduated glass measure. Both measure and scale pans should be kept scrupulously clean.

Apothecaries Weight

Dry.

20

grains

=

1

scruple.

3

scruples

=

1

drachm

=

60

grains.

8

drachms

=

1

ounce

=

480

"

12

ounces

=

1

pound

=

5760

"

Wet.

60

minims

=

1

fluid drachm.

8

drachms

=

1

ounce.

20

ounces

=

1

pint.

8

pints

=

1

gallon.

It must be noted however, that the chemicals are sold by Avoirdupois Weight, in which the ounce and the drachm have other values.

AVOIRDUPOIS WEIGHT.

27 1/3

grains

=

1 drachm.

16

drachms

=

1 ounce.

16

ounces

=

1 pound,

Formulas from foreign sources are generally compounded in grammes instead of grains. The following table for their conversion either way, will be found useful.

1

gramme

=

15.43

grains

2

"

=

30.86

"

3

"

=

46.29

"

4

"

=

61.73

"

5

"

=

77.16

"

6

"

=

92.59

"

7

"

=

108.03

"

8

"

=

123.46

"

9

"

=

138.89

"

1

grain

=

.0648

grammes.

2

"

=

.1296

"

3

"

=

.1944

"

4

"

=

.2592

"

5

"

=

.3240

"

6

"

=

.3888

"

7

"

=

.4536

"

8

"

=

.5184

"

9

"

=

.5832

"