BRIDGMAN, ON INDEX TO SCALE. 
211 
and the arm^ k, fig. 1, may be attached either to the base of 
the stage^ the side of the swinging frame of the bar, or even 
to the bar carrying the body and slow motion. In the latter 
case it will require the style to be longer_, and to have a 
greater range of vertical movement, or a separate adjustment 
may be required to meet the extreme variation in the dif- 
ferent lengths of object-glasses. In every case, whether used 
as a marker or finder, changing the object-glass will necessi- 
tate the papered slide, fig. 2, being first applied to test the 
relative positions, and if the object be in any part of the 
field, it will be easier to allow for this than to alter for every 
occasion, but as most observers work principally with the 
quarter, this will seldom be required. 
By description, even with the assistance of figures and 
diagrams, it is generally extremely difficult to render the 
action of any mechanical contrivance so intelligible as to be 
readily comprehended. In the present case, however complex 
and tedious it may appear, simplicity is one of its greatest 
merits, and when seen in use has invariably won for it instant 
approval. All the minutiae, so far as I can find, have been 
given for its construction, but should any further explanation 
be desired, I shall have much pleasure in replying to any 
communication on the subject. 
On a former occasion, a slight but important omission led 
to an unfavorable conclusion. In his work on the micro- 
scope. Dr. Carpenter, in quoting from a previous number of 
the Journal, our contrivance for scratching circles upon the 
glass, objects to the use of a diamond, as liable to cut or 
star the cover. Omitting to state that the splinter of dia- 
mond should have its point rubbed once or twice on a piece 
of Turkey stone, to give it a scratching instead of a cutting 
edge, might reasonably lead to the above conclusion ; but 
with this precaution no fracture has ever occurred, and we 
have marked test-objects under the thinnest covers as deli- 
cately and as perfectly as can be wished for. 
