QUALITIES OF OBJECT-GLASSES. 
97 
aperture are more suitable to be used in schools and private families where many 
persons use the same microscope, and where, for want of time, in examining a 
variety of specimens, or from inexperience, the necessary attention cannot be 
devoted to adjusting the correction for thickness of glass cover. 
2. Resolving 'power (correct definition being presupposed) may be said to 
stand in a direct relation to angular aperture, and consequently to the obliquity 
of the rays which can be received from the surface of an object. 
To measure the angular aperture of an object-glass^ place the microscope, with 
the body horizontal, on a thin board which turns on a pivot exactly under the 
focus of the object-glass ; set a lamp on a level with it a few yards distant; then 
having directed the body of the microscope so far on one side of the light that 
only half of the field is illuminated, leaving half of it dark, trace a line by the 
edge of the board on which the microscope stands ; now revolve the microscope 
horizontally to the other side of the light till only the opposite half of the field is 
illuminated ; the angle now formed by the edge of the board and the line pre- 
viously traced, is equal to the angular aperture of the object-glass. If the object- 
glass has a very large angular aperture, the line of demarkation between light and 
darkness in the field, will be indistinct, and the experiment must be performed in 
a dark room, with a ray of sunlight entering through a narrow perpendicular slit, 
by which means the exact angular aperture will be more readily determined. 
Without due precaution, errors of several degrees will be made in estimating the 
angular aperture of object-glasses. Adjusting the object-glass for a covered ob- 
ject, will increase the angular aperture of some object-glasses ten or fifteen 
degrees. Extending the draw-tube has a similar effect. Hence, in comparing the 
angular aperture of two objectives, they are to be examined with the same eye- 
piece, similar corrections for glass cover, and the same length of tube. 
Some methods employed to determine angular aperture, reall-y determine noth- 
ing but the angular breadth of the field of view, which is often less than the 
angular aperture for an object in the focus of the microscope. The real question 
to be determined is the angular aperture of the object-glass as ordinarily used hi 
the microscope ; not what is its angular aperture in other conditions where it is 
never placed for practical use. 
3. Flatness of Field. To judge correctly of this quality, object-glasses should 
be tested with an eye-piece which gives a tolerably large field of view. In micro- 
scopes of inferior quality, the defects of the objectives are often concealed by eye- 
pieces so constructed as to give but a very limited field of view. 
4. Depth of Definition. The qualities already enumerated, defining power, re- 
solving power, and flatness of field, may all coexist in the same object-glass, but 
there is another quality so essential to the prosecution of microscopical researches 
of a certain class, and which is generally so little understood and appreciated, that 
we shall dwell upon it more particularly. We refer to the quality which some 
object-glasses possess, in addition to clear definition in the focus, of giving tolera- 
ble definition of parts a little above or below the exact focus. This quality may, per- 
haps, be called Depth of Definition., as it refers to the distance above or below 
the focus where definition ceases, and where objects, by their distance from the 
focus, become invisible. 
The value of object-glasses used for viewing tissues containing cells or vessels 
variously related to surrounding parts, and, in short, for the practical every-day 
work of the microscopist, depends very much on the quality which we have here 
called depth or extent of definition. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
7 
