64 J. W. Bailey on a Universal Indicator for Microscopes. 
the screws in the usual manner and the numbers may be read off 
from the stage scales instead of the Indicator 
The shail mentioned modifications are pxceliens for indi- — 
vidual convenience ; but for the general purposes of science, the 
comparable, transferable, reproducible Indicator, in its simplest 
form, must be preserved ; and it is only in that form that it de 
serves the name, suggested by a friend, of the Universal Indi- 
cator 
Asa proof of the utility and accuracy of the Indicator, and of 
its convenience as a means of scientific exchange, | may state 
~ that numerous mounted slides of minute recent and fossil Dia- 
“toms have been exchanged through the Post Office by Judge 
A. 8. Johnson of Albany, and myself, and that each has found by 
the ordinary as well as modified forms of the Indicator all ‘the 
ells however minute, fragmentary or previously unknown, 
which the athe rhad recorded. Some of these objects were less 
t of an inch in diameter, and yet they were found 
y by means of the hi idicator. 
etermine whether different impressions of the Indicator 
when made on the é kind of paper were comparable, a set of 
objects was editeeds successively by seven different impressions 
made on enamelled cards some of whick were yatranged with the 
ordinary paper centre piece, and othe with. 
lines ruled upon glass. The numbers being reco 
jects when well centred upon one of these Indicators, the: slide wa 
then transferred to each of the other Indicators and each object b . 
ing brought into the field by its recorded numbers, the position was 
carefully adjusted so that the object should be well centred, and 
a record for each copy of the Indicator was thus made. On com- 
paring the different numbers it was found that the coincidence 
was almost perfect, the difference never exceeding } of one of 
thé divisions of the Indicator, an amount which might be quad- 
rupled before an would be thrown out of the field of view 
of my 4 inch objectiv 
The Indicator hevitig been put to so many and such severe 
tests I feel no hesitation in recommending it as a means of scien- 
tific intercourse among observers, and as a means by which col- 
lections of microscopic objects may be registered, arranged, a 
catalogued ; and an index to the whole so made that any particu- 
ar Specimen may be found at will either by the original o 
or any one into — hands the slides and accompanying regis 
ter may at any time come. 
The copy of the iota which accompanies this paper isnot 
given for use with the microscope, as the kind of paper u 
which it is printed is different from that used for the pe a Ine 
dicator, and therefore in consequence of unequal shrinkage a slight 
deviation is produced. The Indicator for use with the microscope 
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