MAGNITUDE AND MINUTENESS. 



completed, the surface, which presents to the senses such brilliant 

 polish, and apparently infinite smoothness, is in reality covered 

 with protuberances and indentations, the height and depth of 

 which cannot be less than the diameter of the particles of powder 

 by which the polish has been produced. 



12. In the detection of matter in a state of extreme comminution, 

 the sense of sight is infinitely more delicate than that of touch. 

 If we rub a piece of gold upon a touchstone, we plainly see the 

 particles of matter which are left upon the surface of the stone. 

 The touch, however, cannot detect them. 



13. In the preceding examples the comminution, however great, 

 cannot be easily submitted to actual measurement. Certain pro- 

 cesses, however, in the arts enable us to obtain exact numerical 

 estimates of a minute divisibility, which without them might appear 

 incredible. If a 'thin tube of glass, being held before the name of 

 a blow-pipe until the glass be softened and acquire a white heat, 

 be drawn end from end, a thread of glass may be obtained so fine 

 that its diameter will not exceed the two-thousandth part of an 

 inch. This filament of glass will have all the fineness and almost 

 all the flexibility of silk, and yet a bore proportional to that which 

 passed through the original tube will still pass through its centre. 

 The presence of this bore may be rendered manifest by passing a 

 fluid through it. 



It has been conjectured that if a filament of this degree of 

 fineness could be obtained of a material that would retain 

 sufficient inflexibility, it might be made to penetrate the 

 flesh without producing pain or injury, inasmuch as its mag- 

 nitude would be so much less than the pores of the integu- 

 ments. 



14. In the application of the telescope to astr nomical purposes, 

 the distance between objects which are present at one and the 

 same time within the field of view of the instrument, is measured 

 by fine threads which are extended parallel to each other across 

 the field of view, and which may be moved towards and from 

 each other until they are made to pass through the objects between 

 which we desire to measure the distance. An experiment, then, 

 which determines the distance between these threads measures 

 the distance between the objects. 



But these threads, being placed before the eye-glass of the 

 telescope, and therefore necessarily magnified in the same manner 

 as the objects themselves, would, unless such filaments were of an 

 extreme degree of tenuity, appear in the field of view like great 

 broad bands, and would conceal many of the objects which it 

 might be necessary to observe. It was therefore necessary to 

 resort to the use of filaments of extraordinary minuteness for this* 

 198 



