FarrantSj on a Micrometer Object-finder. 89 
these scales the numbers against the end of the glass slip^ and 
the lower edge of the object-plate, when the object is in the 
field. This plan may be efficient_, but is of very limited 
applicability, being useful, even to the person who adopts it, 
only with the particular instrument with which the position 
of the object has been registered. 
Other methods more generally applicable have not yet been 
very generally applied. It is therefore not unreasonable to 
presume that all such are deficient in some quality essential 
to their utility. It appears to me that the deficiency is their 
not affording ready means of determining the position of an 
object in the first instance. With most of them it is necessary 
that the '^'^ finder" should be applied to the stage, and be accu- 
rately centred before any use can be made of its scales. This 
is the case with the "Universal Indicator" of Mr. J. W. 
Bailey, described in the Quart. Journ. of Micr. Science,^ vol.iv, 
p. 55, and with the " Finder" of Mr. T. E. Amyot, described in 
the same work, vol. iv, p. 151. On the plan of Mr. E. G. 
Wright, also, it is necessary to " adjust the moveable stage 
exactly square," before the scales are of any use. Now the 
necessity of observing these conditions as preliminaries to the 
use of the finder, must frequently prevent its being employed, 
and that too in cases where the want of it is most felt. It 
must often happen that an observer in examining a glass 
slide meets with some remarkable object, a frustule or valve 
of Diatomacem, for instance, a peculiar crystal, or some par- 
ticular cell occurring either in vegetable or animal structure, 
which he would like to examine more attentively at another 
time ; but not expecting, and therefore not having provided 
for his having occasion to register the position of any object, 
he is not able to do so, or at least, as the first step, before he 
can adjust the stage and apply the scales, he must lose the 
object whose position he desires to register, chiefly because of 
the difficulty of finding it; then, after the necessary ad- 
justments have been made, the object must again be sought 
for. In practice, no doubt, the observer frequently con- 
cludes that this tedious operation may as well bef deferred 
until he shall want the object for further examination, and so 
the registry is not made at all. In using the finder of Mr. 
Tyrrell (see Quart. Journ. of Mic. Science,' vol. i, p. 234), it 
is true that there are no preliminary adjustments to be made, 
yet with it also the object must be lost in removing the glass 
from the stage to place it in the finder, and must be again 
found before its position can be registered. 
The terms latitude and longitude have been already made 
use of in reference to the place of a microscopical object on a 
