Miscellaneous Intelligence, 147 
and hence we have it in our power to measure the extreme limits at 
which the object continues to be visible. 
or the formation of the dioptric images achromatic object-glasses 
might be used; but even where those of the shortest focal length are 
employed, the object whose image it is required to form must be placed 
ata great distance. ‘This would cause various difficulties, and only be 
practicable wilh a microscope placed horizontally—unless, indeed, the 
object selected were very minute, in which case the accurate determi- 
nation of its diameter (from which that of its image must be afterwards 
deduced) would be rendered difficult. 
Small air-bells in a fluid are for this purpose far better. I employ 
by preference a watery solution of powdered gum arabic, which always 
contains numbers of such air-bells originating in the air entangled 
among the particles of the powder. The water employed should have 
stood for a considerable time freely exposed to the air, or been shaken 
up with the air for some time ; for when we use water which is not sat- 
urated with air, the bubbles in ‘the fluid gradually become smaller, an 
subsequent pet ag et as we shall actually find to be the case. 
dro e fluid must then be placed on a clean glass object- 
slide, and — with a good clear mica plate, a ring shaped piece be- 
ing 
of a glass lene would a It will, pate be alw s easy to 
mirror and stage some easily recognized object, e. g. a piece of paper 
or the like. ‘The image is ‘always formed on the under surface of the 
air-bell, which must consequently be brought nearer to the object-glass 
than when it is desired to bring its margins into 
The object whose image is to be ihe subject of examination shos shone 
ward in the space beiween the oe and silage. In some micimecnpes 
fie can hardly be eones® either from the space being g too limited, or in 
ae envelops the space. If such microscopes, in place of a mirror, 
provided with a reflecting prism, the object may be placed opposite 
the side external to the microscope. The instruments best adapted for 
* The following se ol will ee eee this. I brought a printed page of a 
book to such a distance from an air-bell t the length of the image of the whole 
pace was 1- th h milinetre =akoinb 1- con a an inch, and a of the image of each 
etter about 1-480th millim. = 1-12.000ih of an inch. e of their minuten 
these images, oud ae diane 9 light, possessed such danas and , that 
eee SE Dower 54 diameters the whole page was without dificulty 
