132 Chronicles of Science. [Jan. 
improvements have been accomplished by employing specimens of 
glass which allow the minimum of thickness of media to be used. 
The powers with this objective range from 600 to 4,000. The 3, is 
also so constructed, as to admit a large pencil of light, and at the 
same time leave a space between its front lens and the covering-glass 
of the slide sufficient to allow of the examination of ordinary objects. 
The ;!, magnifies with the three eyepieces, 950, 1,700, 3,100, linear ; 
its aperture is 140 degrees; and the thickness of the covering-glass, 
to which it will adjust, is 005 of an inch. 
The 5! of Messrs. Powell and Lealand, as we already said, was 
first made at the suggestion of Dr. Beale, and we can corroborate from 
personal experience the expression made use of by the jurors of the 
late International Exhibition, namely, that it is possible to see by its 
means evidences of structure which are under ordinary powers utterly 
undistinguishable. On looking at an object with the j., after having 
first used the + of an inch, one is immediately struck with the great 
difference in size which it presents. The object looks six times as large 
as it did with the 4 of an inch, and although of course the field is 
darker, it is not nearly so dark as one might be led to expect, consider- 
ing that we are employing a magnifying power of 1,300 diameters. 
In order to see objects distinctly with the y5, it is, of course, 
necessary to use a good light; but it does not require that the light 
should be very much stronger than that ordinarily employed when 
using a quarter. The common microscope-lamp, and achromatic 
condenser, are all that is requisite for the purpose of illumination. 
Like Smith, Beck, and Beck’s 35, Powell and Lealand’s +; object-glass 
is adapted to suit any English microscope. to be used with a covering- 
glass of -005 of an inch in thickness, and to leave a sufficient space 
between the lower lens and the glass to admit of its being employed 
in the examination of ordinary objects. The =; consists, like all other 
good objectives, of eight lenses, two triplets, and one doublet. The 
front one, indeed, measures only :025 of an inch in diameter, and to 
the naked eye looks like a small diamond in its setting. It is, 
however, a vast deal more valuable than a diamond of the same 
dimensions. 
There is great difficulty experienced in the manufacture of these 
lenses ; for they have actually to be ground under a microscope. 
This arises not simply because of their small size, but in order to 
enable the workman to keep the surfaces perfectly level, as a deviation 
of as little as one thousandth of an inch would give rise to both 
spherical, and chromatic aberration. To specify the particular class 
of objects for which the 3; is adapted, is not our present purpose. 
We have now to say a few words regarding the improvements that 
have recently been introduced into the construction of the large micro- 
scope stands. These have for the most part been devised by Mr. 
Ross, with the view of obtaining additional working room for the 
illuminating apparatus beneath the stage, in order to acquire the 
greatest possible angle for simple oblique illumination. This object 
has been accomplished, as will be seen in the figure, by reducing the 
