554 Mr. C. V. Boys [June 14, 



an<yles of incidence. It will be seen that coarse fibres have more 

 bauds than fine, and that the number increases with the angle of 

 incidence of the light. There are peculiarities in the march of the 

 bands as the angle increases which I cannot describe now. I may- 

 only sav that they appear to move not uniformly but in waves, pre- 

 senting very much the appearance of the legs of a caterpillar walking. 



So'uniform are the quartz fibres, that the spectrum from end to 

 end consists of parallel bands. Occasionally a fibre is found which 

 presents a slight irregularity here and there. A spider line is so 

 irregnlar that these bauds are hardly observable ; but, as the photo- 

 graph on the screen shows, it is possible to trace them running up 

 and down the spectrum when you know what to look for. 



To show that these longitudinal bauds are due to the irregularities, 

 I have drawn a taper piece of quartz by hand, in which the two edges 

 make with one another an almost imperceptible angle, and the spectrum 

 of this shows the gradual change of diameter by the very steep angle 

 at which the bands run up the spectrum. 



Into the theory of the development of these bands I am unable to 

 enter ; that is a subject on which your Professor of Natural PhilosoiDhy 

 is best able to speak. Perhaps I may venture to express the hope, 

 as the experimental investigation of this subject is now rendered 

 possible, that he may be induced to carry out a research for which he 

 is so eminently fitted. 



Though this is a subject which is altogether beyond me, I have 

 been able to use the results in a practical way. When it is required 

 to place iuto an instrument a fibre of any particular size, all that has 

 to be done is to hold the frame of fibres towards a bright and distant 

 lioht, and look at them through a low-angled prism. The banded 

 spectra are then visible, and it is the work of a moment to pick out 

 one with the number of bands that has been found to be given by a 

 fibre of the desired size. A coarse fibre may have a dozen or more, 

 while such fibres as I find most useful have only two dark bands. 

 Much finer ones exist, showing the colours of the first order with one 

 dark band : and fibres so fine as to correspond to the white, or even 

 the gray, of Xewton's scale, are easily produced. 



Passing now from the most scientific test of the uniformity of 

 these fibres, I shall next refer to one more homely. It is simply this; 

 the common garden spider, except when very young, cannot climb up 

 one of the same size as the web on which she displays such activity. 

 She is perfectly helpless, and slips down like a bead upon a wire. 

 After vainly trying to make any headway, she finally puts her hands 

 (or feet) into her mouth, and then tries again, with no better success. 

 1 may mention that a male of the same species is able to run up one 

 of these with the greatest ease, a feat which may perhaps save the 

 lives of a few of these unprotected creatures when quartz fibres 

 are more common. 



It is possible to make any quantity of very fine quartz fibre with- 

 out a bow and arrow at all, by simply drawing out a rod of quartz 



