742 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
properties of optical sensitizers, colour screens, spectrographs, direct 
photography in natural colours, &c. As regards standards of light, the 
author gives the preference to the amyl-acetate lamp of Hefner- Alteneck. 
The so-called law of photographic reciprocity is very fully discussed. 
The greater portion of the book, however, is devoted to the action of 
special sensitizers and kindred subjects. 
(5) Microscopical Optics and Manipulation. 
Experiments with a Right-angled Prism.* — Herr W. G. Rontgen 
finds that a right-angled prism may be used in order to demonstrate 
the fact, originally discovered by Helmholtz, that the line of sight of 
the eye does not coincide with the axis of the eye. 
On looking with one eye through the hypothenuse face of a right- 
angled prism with vertical prism edge, an image of the head is seen 
which, unlike that from a mirror, is congruent with the object. The 
author discovered that for any position of the head the line of sight of 
the image from the back prism edge to the pupil in the image always 
passed through the same part, but not through the middle of the pupil ; 
it was always directed to a point from the middle towards the side of 
the nose. 
Further experiments were made by the author with a right-angled 
prism by mounting it on a goniometer with the prism edge parallel to 
the axis of the instrument. When the telescope is directed upon the 
hypothenuse face a sharp image of the cross-wires is seen which only 
coincide with the cross-wires seen directly if the vertical cross wire and 
the prism edge lie in one plane. By rotating the prism the image of the 
cross-wires does not change its position in the least. These observations 
may be made use of in order to set the vertical cross-wire parallel to the 
axis of rotation of the instrument, and also to determine whether the 
prism is exactly right-angled or not. In the latter case, instead of one 
image only of the vertical cross-wire, two are seen which lie so much 
farther to the right and left of the wire seen directly, the greater the 
error of the prism, Since all these experiments succeed equally well 
with mirrors set at right-angles, this observation affords a simple means 
of adjusting two mirrors exactly at right-angles to one another. 
When the vertical cross-wire was rotated through a certain angle 
the image was turned through the same angle, only in the opposite 
direction. When, therefore, the vertical cross-wire was turned through 
an angle a, a rotation of the prism about an incident ray as axis through 
the angle ^ in the same direction brought the image into its original 
position again. 
C6) Miscellaneous. 
The late Mr. G. E. Blenkins, E.R.C.S. — As announced at the 
October meeting of the Society, we have lost one of our oldest and most 
honoured Fellows by the death of this gentleman. He joined the 
Society in 1848, and was active in its service, being Secretary from 
1858-67. We learn some details from the British Medical Journal.f 
* SB. Physikal-medicin. Gesell. Wurzburg, 1894, pp. 53-6. 
t Brit. Med. Journ., No. 1762, 1894, p. 789. 
