ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
107 
Mr. Bulloch now describes it as follows : — “ The improvement con- 
sists in the secondary slide, by which the whole micrometer is movable ; 
avoiding the uncertainty of adjustment made by the ordinary micro- 
meter in getting contact with the cross-hair at zero ; as the amount of 
Fig. 6. 
spring with the best mechanical stage (excepting micrometer stages) 
prevents the cross-hairs being brought gently and perfectly in contact 
with the object to be measured.” 
Mr. Bulloch writes as follows with regard to the additional diagonal 
lines shown in one of the small figs. : — “ From my experience in com- 
paring micrometers and getting the value of divisions of the screw, much 
better results can be reached by intersecting the line on the micrometer 
with the cross line in the eye-piece micrometer; for when using the 
ordinary filar micrometer it is almost impossible to judge the amount 
of space left between the line on the micrometer and the spider line.” 
,C4) Photomicrography. 
Photomicrographs and Enlarged Photographs. — At the December 
meeting of the Society, Mr. J. Mayall, jun., read the following note: — 
The application of photography to microscopy has received in recent 
years so great an impulse from the introduction of the dry-plate processes, 
that the Society has received very large accessions to its collection of 
photographs — especially of such as have been produced with a view to 
illustrating various theories of diatom structure. 
Apart from the question how far it is legitimate to infer the structure 
of such diatom valves either from the images seen in the Microscope or 
from those produced by photomicrography — a question which Prof. 
Abbe’s researches appear to answer in the negative — it appears to me 
that, unless great discretion is used, the after-processes of enlarging 
from photomicrographs may easily lead microscopists astray in giving 
fictitious appearances of contrast in the structure, leading to the belief 
that such strong images must necessarily represent what was visible 
in the Microscope. It is well known that photomicrographs frequently 
give a very erroneous rendering of the different tints seen on delicate 
