242 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
incandescence gradually, but when attention bas been paid to this point 
it will last from 60 to 70 hours. 
Photomicrography at the Photographic Jubilee Exhibition at 
Berlin, 1889. — Dr. R. Neuhauss, who gives his impressions of the 
photographic exhibition, awards the first place. in the microscopical class 
to the photographs exhibited by the Berlin Hygienic Institute, These 
are principally the work of Dr. Koch and Dr. Pfeiffer. The latter 
showed the flagellate miero-organisms, some of which have appeared in 
the ‘ Atlas of Bacteriology ’ of Fraenkel and Pfeiffer. The Institute 
also showed a very interesting series illustrating the progress of 
microscopical photography. 
Schultz-Henke showed two photographs taken from the same pre- 
paration — one with the ordinary dry plate, the other with the eosin-silver 
plate (spinal cord x 30). The latter photograph showed more details, 
but it is possible that the dry-plate process was not shown at its 
best. 
Max Hauer exhibited a series of photographs from his ‘Atlas of 
Vegetable Anatomy.’ The photographs, which were very large, had 
been taken with relatively low powers. The size had been attained by 
means of a' large camera or subsequent enlargement of the negative. 
The defect of this procedure is that the photographs show diffraction 
lines, a defect possibly inseparable from the method. 
The foregoing afford a good illustration of the exhibits, but there 
are several others mentioned by the author, including an album of his 
own work. 
(5) Microscopical Optics and Manipulation. 
Method of Detecting Spurious Diffraction Images.f — Mr. E. M. 
Nelson writes : — In a previous paper I gave as my opinion that certain 
alleged diatom gratings, of double fineness and either above or below 
the original structure, were spurious, because they were caused by the 
action of an over- or under-corrected Microscope objective on the 
diflraction-spectra. 
I now show how a test may be applied to determine whether these 
structures are entities or only diffraction-ghosts. The test will suit 
equally well other objects which yield a similar arrangement of 
interference conditions. 
Set up the Microscope and adjust tube-length, &c., so that the best 
view is obtained of say the upper fine grating in Fleurosigma formosum, 
the reality of which is required to be tested. By means of the fine- 
adjustment the distance between this fine grating and the original coarse 
grating is accurately measured. The draw-tube of the Microscope is 
now lengthened one inch or more, and the distance between the two 
gratings is again measured. If this last measure agrees with the former 
measure, the grating is in all probability an entity, but if the measure 
with the long tube exceeds that with the original adjustment, then the 
fine grating is an optical ghost. In the case of an under-corrected 
objective, with a fine grating below a coarse grating, it will be necessary 
* Zeitschr. f. Wiss. Mikr., vi. (1889) pp. 273-7. 
t Note read 10th April, 1889. Cf. also Journ. Quek, Micr. Club, iv. (1890) 
pp. 55-6 (1 fig.). 
