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III . — Measurements of the Mailer Probe-Platte. 
By J. Edwards Smith, Esq., Ashtabula, 0., U.S.A. 
Some twelve months since my friend Professor Edward W. Morley, 
of Hudson, 0., at my request made careful and accurate measure- 
ments of his Moller Platte (Ho. 258), and kindly forwarded to me 
the results obtained. I have cross-questioned pretty severely some 
of the figures as given by Professor M., by “ throwing down ” the 
images on paper with camera lucida, and comparing the various 
markings each with the other graphically, and find the Professor’s 
results to harmonize very nicely. 
In making the measurements Professor Morley used the superb 
Houghton and Sims’ micrometer, belonging to the Hudson Equa- 
torial. The objective was a very fine Tolies’ T gth. Monochromatic 
sunlight was also employed. 
With pleasure I forward you the results obtained by Professor 
Morley, as per table annexed. 
I have had occasion to examine several of these Probe-Plattes of 
Mdller, and found them all singularly even as to the “ markings.” 
This being the case, the Platte becomes at once valuable as a stage 
micrometer, using the camera and tracing the markiDg on paper — 
hence these values would at once be obtained by consulting the 
table. The whole process is so plain that it is quite unnecessary 
for me to go through the details. It will sometimes be better to 
trace several of the markings from the camera, as by example 
given. 
Take shell No. 3 ( Nav . lyra), and throw, by camera, its markings 
on paper. Selecting now the lines best tabulated and tracing off 
the space occupied by six of these, the distance obtained should 
equal the distance similarly obtained from shell No. 1 ; or we may 
trace from No. 1 the distance occupied by six hexagons, and dividing 
this by six graphically, get a more correct result than could be 
obtained by measuring a single hexagon. 
After a little practice the observer becomes expert with the 
camera, and will be able to deal with the finer shells. Mono- 
chromatic sunlight will now be found of advantage. 
Again, the observer may subdivide the distances given by the 
coarser shells, and thus obtain finer comparison scales, i. e. taking 
the distance already obtained from the tracings of T.favus (No. 1), 
and with a fine lithographic pen divide this into three parts ; each 
of these parts will represent the -nnrrrth of an English inch ; all 
this is. plain enough. 
Professor Morley assures me that he can readily measure 
N. crassinervis (No. 18) of the Platte with his T Vth, using lamplight. 
When we bear in mind the high magnifying power of the H. and 
S. micrometer, and also the delicacy required for this kind of work, 
