POWER OF SHORT SPACES. 



37 



made on glass, or in tin-foil, is held at the end of a darkened 

 tube about two inches long:, and examined bj placing the eye 

 at the opposite end, and looking either at a white cloud or a 

 window blind on a sunny day, or at a lamp with a ground glass 

 shade, it appears altered in size, shape, and transjxircncy . 



In order to illustrate this, and to give an idea of the image 

 thus formed on the retina of the eye, let AA (fig. 9) be one of 

 these apertures fixed in the end of a darkened tpbe T, and let 

 AC, AD be rays of light admitted through it. This light will 

 diverge in lines AC and AD, and form an image CD at the 

 bottom of the eye. 



If tlie same aperture be removed a few inches farther from 

 the eye, it presents nothing remarkable, and in no wise differs 

 in appearance from what we know to be its real form, namely, 

 a transparent line of exceedingly small dimensions. Bat if it 

 be again made to approach the eye, it will appear, first, to be 

 much magnified ; secondly, to have lost its rectangular o\itline, 

 and to become rounded at either extremity ; and tliirdly, to 

 he traversed by dark bands which take a direction parallel 

 to its long axis, as sliown in figure 9. 



Fiir. 9. 



These glass covers are sold by the ounce, and are cut into squares or 

 circles of various sizes for the convenience of mounting. The Indian ink 

 might be painted on the glass by hand; but, after having made several 

 gross of such black discs, the author of these ]iapers strongly recommends 

 a little instrument wliicli, although constructed for a totally dififerent 

 purpose, answers most admirably for this. It is the invention of Mr. 

 Shadbolt, and is described and figured in the second edition of Quekett's 

 ' Treatise on the Microscope,' p. 289. This instrument is nothing more 

 nor less than a miniature horizontal turning lathe, which is worked by 

 the finger, and by which, with the assistance of a carael's-hair pencil, the 

 ink may be laid on in circles with the greatest nicety and expedition. 

 When dry the narrow line is erased with a finely pointed and slightly 

 moistened one-nibbed quill ; or, what is better, a style of brass drawn 

 along a flat ruler. Wlien tin-foil is used instead of glass, it may be held 

 on a piece of smooth flat lead ; an aperture of the required size can then 

 be cat completely through with the point of a penknife. 



