SCIENTIFIC SUMMARY. 
323 
the field. Beneath the slit is an arrangement for holding an object of irre- 
gular surface or dense substance. The stage has a concentric movement, so 
as to permit the object to rotate, and enable the image of the slit to pass 
through it in any direction. The direct-vision prisms consist of three flint 
and two crown, fitted in a box screwed into the end of the microscope. By 
means of a pin they are thrown in or out of action. The object-glass screws 
on in front of the prism-box. By taking the illumination from the sky or a 
white cloud, Fraiiuhofer’s lines are visible, and by direct simlight they are 
seen in great perfection ; the dispersion is sufficient to cause the spectrum to 
cover the whole field, and the achromatism of the lenses being nearly per- 
fect, the lines from n to G- are practically in the same focus. A double- 
image prism near the slit enables two spectra to be seen, oppositely polarised, 
and the variations in the absorption lines are at once visible. A Nicol’s 
prism as polariser, and another as analyser, can be connected, and these 
enable the brilliant colours shown by some crystalline bodies, when seen by 
polarised light, to be examined. 
PHOTOGEAPHY. 
Cracking of Negatives . — The cracking of the thin film of collodion which 
forms the negative is a source of annoyance and loss to many photographers, 
and there are few artists of much experience who have been wholly exempt 
from it. A few weeks ago this subj ect was revived at a meeting of the 
London Photographic Society by a lady whose name is well and favour- 
ably known in connection with the artistic development of photography, 
Mrs. Julia Cameron, who had discovered that a large number of her best 
negatives had their films seriously damaged by means of a delicate net- 
work of cracks. This evil, although serious, is easily prevented and 
its effects equally easily cured. Dampness of the atmosphere is gene- 
rally understood to cause the reticulated crackings ; hence, to prevent the 
atmosphere from having any efiect of this kind, the negatives, when not in use, 
should be kept in packets separated from each other by means of one or more 
layers of blotting-paper. When thus packed and wrapped round with 
paper sized or varnished, so as to be comparatively waterproof, it may be 
safely assumed that the negatives will never crack in the manner described. 
A plate-box of the usual form is objectionable on account of each plate 
being exposed to the action of the atmosphere by which it is siuTounded. 
When a negative has already become cracked, a soft pad of cotton wool 
should be charged with some fine lamp-black, and rubbed all over its 
surface. The crackings are thus filled up so as to render their previous 
existence incapable of being detected. Whatever may be thought of the 
soundness of the principle of this remedial measure, there can be no doubt 
whatever as to its excellence in actual practice. 
The Morphine Process . — An American photographer, in experimenting with 
morphia as a preservative agent for dry plates, has found that by means of 
the following preparation, which keeps w’ell, plates may be prepared that 
will yield fine negatives for many months after they ai’e made. Mix toge- 
