394 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the same as that employed by Dr. W. C. Borden, as described in the 
April number of this Journal, p. 262. 
Photomicrograms for Purposes of Instruction.* — Herr K. Karg re- 
commends the use of photomicrograms for purposes of instruction in 
preference to the drawings used hitherto. The want of colour is the 
great drawback, but in spite of this, the beauties of a good photomicro- 
gram are so manifest as to make it preferable to any drawing, even when 
the latter is really exact. 
Hints on Photomicrography.! — Those interested in photomicro- 
graphy should consult Mr. E. M. Nelson’s Presidential Address to the 
Quekett Microscopical Club, where this experienced and expert worker 
gives some of the results of his work. 
(5) Microscopical Optics and Manipulation. 
An Instrument of Precision for producing Monochromatic Light 
of any desired Wave-length, and its Use in the Investigation of the 
Optical Properties of Crystals*! — Mr. A. E. Tutton has devised an in- 
strument for use in the investigation of the optical properties of crystals, 
which enables the whole field of any optical instrument whose aperture 
does not exceed 2 inches to be evenly and brightly illuminated with 
monochromatic light of any desired wave-length. 
In this instrument, which resembles 1 a compact spectroscope in ap- 
pearance, the exit slit is fixed, while the dispersing apparatus is rotatory. 
The apparatus consists of a strong stand, carrying a fixed horizontal 
circle about which two exactly similar optical tubes are capable of 
counterpoised rotation. These tubes carry at the ends nearest the centre 
of rotation lens-combinations of 2 in. aperture and 9 in. focal length, 
and at the other ends a special form of slit with jaws capable of 
equal movement on each side of the central line, which thus remains 
fixed. The lenses of the combinations are not cemented together, but 
held in metal frames, so that no alum cell is required to protect them 
from the heat rays. 
The dispersing apparatus, consisting of a single 60° prism, with 
large faces, in. by 2 A in., is carried by a divided and rotating circle, 
parallel with and above the fixed circle. 
One of the optical tubes is chosen as collimator, and sunlight is 
reflected along it from a mirror attached to a tapped annulus projecting 
from the slit frame. The other tube is converted into a telescope by 
the similar attachment of one of three eye-pieces.* By the proper adjust- 
ment of prism and telescope, it is then possible by rotation of the prism 
to bring the whole of the spectrum past the exit slit, and to mark the 
readings of the prism when prominent solar lines are adjusted between 
the edges of the slit. The mirror and eye-piece are then removed, and 
by illuminating the receiving slit with any artificial source of light, 
light of any wave-length may be made to issue from the exit slit by set- 
ting the prism to the reading corresponding to that wave-length. The 
issuing light is diffused by means of a screen of ground glass of fine 
texture, which is contained in a tube of 2 in. diameter attached to the 
* Verhandl. d. Anatom. Gesellsch. 7. Yers. in Gottingen vom 21-24. Mai 1893. 
See Zeitschr. f. wiss. Mikr., xi. (1894) pp. 25-6. 
t Journ. Quek. Micr. Club, v. (1894) pp. 348-65.. 
X Proc. Roy. Soc., lv. (1894) pp. 111-3. 
