498 Production, Properties, and Uses of the Finest Threads. 



spectra which would be brighter than those given by a 

 corresponding grating of ordinary construction, because not 

 only is all the light which falls on the surface brought to a 

 series of linear foci forming the bright lines instead of being 

 half removed, as is usually the case, but the direction of the light 

 on reaching these lines is not normal to the grating as usual, 

 and therefore in the direction of the central image, but 

 spreading, and thus in the direction of all the spectra. I 

 picked out a quantity of glass fibre not varying in diameter 

 more than one per cent., and made a grating in this way 

 covering about one eighth of an inch in breadth. This not 

 only showed three spectra on each side, and a quantity of 

 scattered light, but all the spectra were closely intersected by 

 interference-bands, such as are seen when a Newton's ring of 

 a high order is seen in a spectroscope. This is probably due to 

 a cumulative error in the position of the fibres, for they 

 were spaced by being pushed up to one another with a 

 needle-point, or to light passing between the fibres in a few 

 places where dust particles keep them apart. 



A diffraction-grating made of these fibres, spaced with a 

 screw to secure uniformity, and of a thickness equal to the 

 spaces between them (and one of 1000 lines to the inch could 

 be easily made) would be far more perfect for the number of 

 lines than any scratched on a surface ; that is, for investigation 

 on the heat of a spectrum, such a grating would be preferable 

 to a scratched one, as there is no uncertainty as to the grating 

 or to the substance of which it is made *. If the transparency 

 of the fibres interfered they could be rendered opaque by 

 metallic deposit without visibly increasing their diameter. 



There is one use to which the fibres of quartz tailing-off to 

 a mere nothing might be applied, namely as a test-object for 

 a microscope. Theory shows that no microscope can truly 

 show any structure much less than j -g o*o o o i ncn ? or divide two 

 lines much less than this distance apart. Natural bodies such 

 as Diatoms &c. have this advantage, that they can be ob- 

 tained in any quantity alike, but no one knows what the real 

 structure of these may be. Nobert's bands.are good in that 

 we know the number of lines in any band, but as to the indi- 

 vidual appearance of the lines and spaces it is impossible to 

 say anything. These fibres have the advantage that we have 

 a single thing of known form, which tapers down from a 

 definite size to something too small even to be seen. Though 

 it may be possible to calculate the size from the appearance 

 of the fringes, yet whether the size is known or not, at each 



* See < Heat,' by Prof. Tait, p. 268. 



