PROCEEDINGS OF THE SOCIETY. 
151 
all. It Lad no movable throad so far as he could see ; the scale moved 
sometimes, but not the web. 
Mr. Mayall said the apparatus was so shaky that ho supposed it had 
met with an accident. The general construction reminded him of the 
designs of the Continental screw-micrometers, and also of the screw 
mechanism frequently employed on the Continent for stage movements, 
centering, &c., in most of which unnecessarily long and thin screws were 
applied, which were very liable to be bent, and to become loose in 
their sockets. He thought the defective condition of Mr. Dowdeswell’s 
micrometer should serve as a warning to opticians generally of the 
error of making screw-axes too long and thin, especially those having 
milled heads, and, consequently, intended to be moved by hand. In 
all the high-class screw-micrometers and similar mechanism, the 
actuating screws and their bearings were made large and substantial, 
with a view to securing accuracy of movement, durability, and freedom 
from flexure. He might mention particularly that in examining a 
large number of Microscopes by different makers, he had observed 
that the centering screws of the mechanical substages were generally 
too slight, and were provided with such short sockets that they were 
very liable to become shaky. These were points of importance in the 
construction of Microscopes and accessory apparatus. 
Mr. Mayall said he had prepared a short note for publication in 
the Journal (see ante, p. 107) upon a matter in connection with photo- 
micrography, which he thought the Fellows of the Society would 
agree with him called for some protest — the practice of sending 
photographs there as specimens and illustrations without at the same 
time stating the details of the process by which they were produced. 
On submitting the note to Prof. Bell, it had been thought advisable 
to deal with it as a communication to the Society, in the hope that 
it might lead to useful discussion. Before inviting discussion, he 
said his attention had been specially drawn to the subject by the 
fact that at one of their recent meetings a photograph of P. angulatum , by 
Dr. Yan Heurck, had been exhibited, and had elicited from Mr. Frank 
Crisp the observation that it was <c remarkable.” In making that obser- 
vation, Mr. Crisp certainly supposed that he was criticizing a photo- 
micrograph pure and simple — produced directly with the Microscope — 
and on the fact of that supposition the observation was doubtless 
fully justified. On examining the print again more closely, he (Mr. 
Mayall) was inclined to suspect that it was not really a photomicro- 
graph, but merely an enlarged copy of one, and that a very large 
part of the strength of the image was probably due to the copying process 
employed. He therefore applied to Dr. Yan Heurck for information 
and found that the print was an enlarged copy, as he had suspected. He 
thought that was an important example to cite of the erroneous value 
that might be given to a photographic rendering of a microscopic object, 
in consequence of the full and proper data not having accompanied the 
photograph ; for assuredly, if Mr. Crisp had been aware of these data, 
his commendation of the photograph would have been modified. 
Mr. Mayall explained that the limit of useful magnification for the 
