148 Miscellaneous Intelligence. 
the manipulation which we are describing are, however, those whose 
illuminating apparatus consists of a mirror and converging lens, which 
i wh. ens being removed from the ring 
which supports it, the object is substituted in its place. The relative 
o 
ny 
a 
a. 
a) 
re 
(9°) 
Of course it is impossible to measure directly the dimensions of this 
most minute visible image, for our best micrometric methods will here 
fi no avail. Yet their size may be estimated with extreme 
accuracy in the following manner. At the same distance from the air- 
bell and in place of the object used, substitute another body; such as a 
piece of card, of 4 to5 centimetres = 1 2ths to 2 inches diameter, which 
has been exactly measured. Let this be now again measured (by some 
more minute object, but to divide its true diameter by the figure ex- 
pressing the diminishing power. 
For example, let the true diameter of the greater object be 5 centi- 
metres =to 1-969 English inches, and the diameter of its image = 
32:2 micromillimetres,+ = 00127 English inches, then the figure ex- 
pressing the amount of diminution will be 3;95% = 1553 very nearly. 
round or long thread-like form, grains of pear sago, or vegetable bod- 
jes, such as mustard-seed or the pollen-granules of many plants, hairs 
of animals, metailic wires, &c., may be employed. Small round open- 
ings and chinks may serve for the determination of the visibility of pos- 
ilive images of light. In the last case, care must o rse be taken, 
by means of suitable screens, to shut off all licht except what passes 
* See translation from Het Mikroskoop in Monthly Journal of Medical Science, 
June, 1852, p. Seba seq. 
+ The micromillimetre is equal 1-1000th millimetres = -0000394 English inches. 
See Monthly Journal, June, 1852, p- 456, me 
