280 The Rev. E. Craig ow the Configuration of the Scales 



There are several notices in the writings of Sir David Brew- 

 ster, Dr. Goring, and Mr. Pritchard on the lines upon the 

 scales of butterflies' wings which appeared to me to leave the 

 subject still in an unsatisfactory state. They speak of longi- 

 tudinal lines, of cross striae, and of two sets of diagonal lines 

 besides, visible on each scale of the P. Brassica. It is ad- 

 mitted that the longitudinal and cross lines may be discovered 

 with tolerable facility; but as to the diagonal lines, notwith- 

 standing the distinct averments of Dr. Goring and Mr. Prit- 

 chard, a doubt still exists as to their actual existence. Dr. 

 Goring observes that they may be seen as distinctly as the 

 ruled lines in a copy-book, yet admits that there is " an in- 

 explicable mystery about them." Sir David Brewster expresses 

 with some hesitation, in opposition to such authority, his con- 

 viction that the diagonal lines are an optical illusion. He 

 states of the whole linear appearances of the scales, that they 

 are none of them lines, but a succession of teeth arranged in 

 lines, and from the great number of lines forming the sides 

 of the teeth they appear dark. Each fibre has teeth on each 

 side of it, and the teeth of one fibre lock into the spaces be- 

 tween the teeth of the adjacent fibres : "the (longitudinal) lines 

 therefore are only appare7it lines, being composed of a suc- 

 cession of interlocking teeth ;" and " the diagonal or oblique 

 lines are optical illusions, from the accidental alliguements of 

 the sides of the teeth in different grooves when similarly il- 

 luminated by oblique rays." Having been long familiar with 

 these objects, and not being quite satisfied with the .above 

 statements, I submitted a great many of these scales to 

 lengthened and intense observation. I examined them through 

 instruments, to the use of which I am very much accustomed, 

 and which, as all microscopic observers are aware, is of great 

 moment for the detection of fallacious appearances : the one 

 is a good achromatic instrument by Chevallier, the other in- 

 cludes a very good Wollaston's doublet, a single garnet lens 

 by Adie of the -^-^ of an inch focus, and another by Blaikie 

 of the j\j. It becomes me, in stating that I have arrived at 

 a different result from these accurate observers, to speak with 

 the greatest diffidence. I shall be thankful if anything stated 

 by me shall serve only as a hint for their further considera- 

 tion. 



The distinct impression upon my mind, after a long series 

 of observations, is that each scale is a film, regularly ribbed 

 or divided by longitudinal fibres which are thicker than the 

 rest of the film, like some corded muslins; and that each 

 thinner portion of the film between the ribs is crossed in a 

 slightly curved direction by still finer fibres, which become 



