1854.] OF THE ROYAL INSTITUTION. 403 
structure of certain test objects, such as the siliceous shells of 
diatomacee, was explained to be totally distinct from the mere 
increase of the amount of light transmitted. Mr. Brooke offered 
an hypothesis as to the structure of these objects, from which it 
would follow that that structure would be rendered visible by 
oblique rays alone, and the necessary degree of obliquity would 
depend on the smallness of the elevations on the undulating 
surface of the shell. This view was thus shewn to be highly pro- 
bable ; a specimen of the Pleurosigma formosum (first found by 
Mr. Brooke at Walton-on-the-Naze) was viewed under a half 
inch object-glass by Ross, and an achromatic eye-piece of high 
power, (which was stated to be unquestionably superior to a deep 
Huygenian eye-piece) ; when an opaque disc was interposed 
between the object and the centre of the object-glass, which cut 
off a large portion of the central rays, the diagonal rows of dots 
were still distinctly visible; but when the marginal rays were 
stopped out by a diaphragm, although a much larger quantity of 
light was admitted than in the former case, the markings were 
entirely lost. 
In order to render visible the more difficult objects of this class, 
glasses of large angle of aperture have been constructed, but their 
employment is much limited, owing to the greatly increased 
difficulty of correcting the aberrations, under any given circum- 
stances of the transmitted pencil of light, and consequently the 
small amount of correction, that is, of adaptation to altered cir- 
cumstances, that they admit of. From investigations which he 
knew to be in progress, the Lecturer expressed a hope that by due 
adjustments of the illuminating pencil, the most difficult test- 
objects would be rendered equally visible under object-glasses of 
moderate aperture, which are much more generally useful. 
Mr. Brooke then alluded to the preposterous reputed angle of 
aperture of certain foreign object-glasses, viz. 172°, and explained 
the fallacy of the ordinary method of determining that angle; 
which consists in viewing through a microscope the light of a 
lamp placed at a few feet distance, and moving either the light or 
the microscope, so as to traverse the entire angular distance 
through which the light is visible. In this method the course of 
the rays is contrary to their usual course, and oblique pencils may 
be brought to an imperfect focus at the back of the object-glass, 
and produce a glare of light, but which meet ata greater angle 
than the extreme rays that can enter the object glass from the 
field of view, and which consequently are the extreme available 
rays. 
A very perfect instrument for measuring the angle of aperture, 
designed by Mr. Gillett, was then explained: this consists of two 
microscopes, the optical axes of which may be adjusted to coinci- 
dence. One of these is attached horizontally to the traversing 
