62 J. W. Bailey on a Universal Indicator for Microscopes. 
The power of the objective employed in determining the por 
sition of an object for registration, should always be the highest — 
which can be conveniently employed ; while in searching for au 
object already recorded, a power lower than that employed in the — 
spent may be used. The object then must be in the field — 
of view, and would be at the centre but for slight errors in man- 
Fo tion, or the want of perfect adjustment in “the mountings of — 
the object-glass. Care should be taken to bring each object ac- — 
f wi ew of my 4 inch objective made . Spencer, includes two 
aividiogs of the Indicator, and hence an error of nearly one divis- 
ion nigh t be made in placing a slide upon the Indicator by means 
ecorded numbers and yet the object would be found in 
happe’ 2 that in transferring a slide fram one Indicatot 
dicator with the slide placed at any recorded position until the ob- 
ject comes into the centre of the field of view, rte “secure the In- 
dicator to the stage in this new position, and all other object 
corded by the same Indicator ought to be brought to the centre 
the field of view by means of the numbers as registered. : 
’ e convenience of the Indicator for individual use may be f 
injeenod by several slight changes. One of these consists in re~ 
moving the paper centre piece, and replacing it either temporarily 
or permanently by a glass plate bearing lines at right angles to 
each other ruled very lightly with a diamond point, and so adjnsted - 
as to coincide with the ig trices of C Dand EF through the 
centre. For all but the highest powers there is no objection to 
having these excessively minute lines permanently beneath the 
centre of the Indicator as they do not perceptibly interfere with — 
the light, and it is convenient to have them always in place. 
They: can be ruled upon a piece of mica or thin glass cemented — 
to the back of the Indicator, or the latter may itself be cemented — 
to a piece of plate glass and the central guide lines then carefully 
rul Even for the highest powers these lines can be use’ in 
recording the position of objects, which can then be found oe 
study by using an Indicator of the ordinary form. By a proper 
arrangement, a moveable stage, with screws for vertical and hori- | 
zontal motions, may be graduated so as to correspond to the In- 
dicator, and yet preserve all the advantages of accurate adjustment — 
which the screws afford. For this purpose it is necessary to ob- 
