the Microscopical Society, 
83 
chord of 170° is more than '996 of the diameter of a circle, 
we may be certain that if the extreme limit has not already 
been reached, its further extension will scarcely be appreciable. 
To correct the aberration of these glasses as far as possible, 
and to bring them to the neatness of definition that has been 
attained in those of more moderate aperture, must now be the 
aim of our scientific opticians. 
A very useful addition to the mechanical arrangements of 
the microscope has been contrived by Mr. Brooke. In former 
days, when our objectives were single lenses, it was usual to 
set four or six of them in a wheel, by turning which the 
power could be changed in a moment. Our present object- 
glasses, consisting generally of three achromatic combinations, 
requiring to be set in tubes of some length and thickness, 
cannot be compressed into so small a space. Mr. Brooke has, 
however, effected the same purpose to the extent of two powers. 
To the nozzle of the microscope an arm is screwed, projecting 
in front, and carrying a pin on which a bar revolves, to each 
end of which an object-glass is screwed. Either of these, by 
rotating the bar, can be brought under the body of the instru- 
ment, while the other is carried beyond the stage so as to be 
quite out of the way. Object-glasses of one inch and one- 
quarter inch mounted in this way are found to be very con- 
venient when pursuing microscopical researches ; the one to 
take a general view, and the other a particular one, of the 
object under inspection. 
Mr. Brooke also exhibited a neat little contrivance for con- 
verting a pocket eye-glass into a table microscope. Two 
straight square pieces of brass are halved into each other, and 
the pillar on which the eye-glass slides screws into the inter- 
section, the straight pieces forming the foot. The whole 
makes a useful stand, and packs into something smaller than 
an ordinary spectacle-case. 
Mr. Ross has constructed a very comprehensive microscope- 
stand, furnished with right-lined and circular motions, not 
merely to the stage but to what may be called the sub-stage^ or 
that part which carries the different illuminators for trans- 
parent objects. All these motions being effected either by 
pinions or screws, the various adjustments are made with 
great comfort to the observer. The instrument is heavy, and 
the quantity of excellent work in it necessarily renders it 
somewhat costly. 
Messrs. Smith and Beck have adopted an improved method 
of attaching the object-glass to the body for the purpose of 
preventing the excentricity which is frequently caused by the 
imperfection of the screw. They have also carried the same 
