78 THE NORTHERN MICROSCOPIST. 
pictures—I ask you to examine this photograph of the blow fly’s 
proboscis taken with one of Swan’s plates, and say whether any 
more detail could have been shown. Look at the delicate hairs, 
the teeth, apply a lens to the false trachea or suctorial tubes and 
say whether, in your opinion, a wet plate would have shown more 
than this. [ This photograph has been reproduced by the Wood- 
bury type process, and is given as Plate V. in the present number. | 
There are many kinds of dry plates in the market which are now 
largely prepared from a gelatine emulsion. Of those I have tried, 
the preference is given to Swan’s and to those made by 
Wratten & Wainwright. They are made up in packets of one 
dozen each, for which is charged three shillings if of quarter plate 
size, or measuring 4} x 3+ inches. 
Do not over expose these gelatine plates, for if you do you will 
get nothing but thin and misty images. From some cause those 
exposed to gas or lamp-light do not commence to develop so quickly 
as those exposed to sunlight, but if time is given and they are not 
over exposed, perfect pictures may easily be obtained. ‘The time 
of exposure in the camera depends upon the quality of the light, 
but more perhaps upon the nature of the object than anything else; 
for instance, the section of a kidney of a rat required twenty 
minutes exposure with the argand gas and bull’s-eye condenser, 
while a crystal of glass under the same conditions of light and 
amplification required only three minutes. 
For ascertaining the exact exposures and the corrections required 
for the difference in actinic and visual foci, I strongly advise the 
beginner to keep a bound record of his failures as well as prints 
from a// his negatives, with full particulars respecting them ; cart 
are great helps in photographing new subjects. 
The following actual exposures with various objects may ie 
found useful to others, the plate being at a distance of 36 inches 
from the object :— 
OBJECTIVE. SUBJECT. EXPOSURE. 
Four-inch...... Wing,of: Blow Fly......5: ese 15 seconds. 
Four-inch ...... Segment of Blow Fly............ 30.9 Wiiy 
Two-inch....... Wing of Blow ‘Fly... eee 26 UR 
Two-inch....... Proboscis of Blow Flyx::c:aee Gores 
One-ineh, 3.4.08 Wing of Blow Fly.:....2.. eae Go 
Oneanchic3 Proboscis of Blow Fly............ 2 minutes. 
Half-inch....... Glass Crystal. ». 0.00...) 23a S 7G 
Half-inch....... Section’ of Frog’s Lung.: aie Zon iss 
Quarter-inch..;Glass ‘Crystal..........434-0e eee TO* Ny 
Quarter-inch...Arachnoidiscus Ehrenbergii...... Ley ee 
Quarter-inch...Peronospora Infestans............ 10 ey 
Quarter-inch...Section of Pine Wood............ 267 
Eighth-inch....Cuticle of Esparto..............000 Ao: 
Eighth-inch....Pleurosigma attenuatum ...... . 40 
