INDUSTRIAL PROGRESS DURING THE YEAR 1875. cxxxix 



is an important paper by Dr. Pigott on the invisibility of 

 minute bodies, subtending a sufficient visual angle to be 

 readily seen if properly defined. This invisibility depends 

 upon several causes, which are examined, and the results 

 given in detail ; and, first, for minute gas bubbles (vacuum 

 bubbles ?) in plate glass : these examined by the horizontal 

 microscope, placed opposite the window, give a very perfect 

 picture of the prospect in miniature; the field of view pre- 

 cisely three fifths of the diameter of the bubble, and the 

 marginal band one fifth the same for all objectives, tohat- 

 ever be the aperture. Not so with a solid spherule of the 

 same size and same glass, for, first, the marginal band in- 

 creases in breadth from nothing till it occupies the whole 

 spherule as the aperture is diminished ; and, second, the de- 

 gree of aperture at which the band first appears varies with 

 the refractive index of the bead. If a small spherule be 

 formed by melting the end of a fine glass thread, and exam- 

 ined under the microscope, using the plane mirror before a 

 window, a minute image of the window appears, surrounded 

 by a black annulus, which Dr. Pigott calls the " black test- 

 band," it will be found that for the same aperture the breadth 

 of the black ring is exactly in the same proportion to the di- 

 ameters of the spherules; the angular aperture is at once 

 shown by the breadth of the picture displayed within the 

 spherule. On increasing the aperture the picture becomes 

 larger and larger, until with a large aperture the ring is 

 attenuated exceedingly; and upon diminishing the aperture 

 exceedingly, the test-band widens so much that only a mi- 

 nute picture is left in the centre. It is evident that this 

 test-band has a remarkable effect upon definition. If we 

 are observing minute spherules in a mass, with excessive 

 aperture, the bands become almost invisible, the forms of 

 closely packed beading, if refractive and transparent, can 

 not be descried, and if there be brilliant illumination, the 

 forms under inspection are completely obliterated. 



The Rev. Dr. Edwards, of St. Chad's College, Denstone, 

 England, proposes for the unit of linear measurement in 

 microscopical observations the wave - length of, say, yellow 

 light, or, perhaps better, of orange ; in this latter case we 

 would have 1,500,000 wave-lengths = 36 inches, and, in 

 round numbers, 1 wave-length = the 40 uoo of an English 



