of Butlerjiiei Wings, as exhibited in the Microscope. 28 1 



closer in their arrangement as they reach towards the serrated 

 end of the scale. I conceive that the dark longitudinal striae 

 are not formed by a close combination of interlocked teeth, 

 because I have observed many lacerated scales, which invari- 

 ably split and tear up tlie middle of the dark line, and in 

 every such case exhibit a smooth edge without any appear- 

 ance of teeth. The scales tear down these dark ribs in the 

 same way as some leaves tear easily down the nerves. More- 

 over it is easy to arrange the lens so as to leave the other 

 parts of the film dark, and the light passing through these 

 longitudinal ribs or striss in the same way as through the 

 nerves of a leaf. That the cross striae are thinner than the 

 longitudinal is evident, because when the instrument has been 

 arranged for the most distinct vision of the longitudinal lines, 

 a slight and delicate adjustment of the object nearer to the 

 lens throws the longitudinal lines out of the focus and brings 

 out the cross striae distinctly. Now it is between these two 

 adjustments that the range exists for accurate investigation of 

 the whole phaenomena. The cross ribs or striae may thus be 

 traced running up into the longitudinal, and rising with a 

 curve out of the interstitial furrow. 



I would observe also that in consequence of this cross- 

 ribbing of the scale, the longitudinal lines are not the same 

 height all along the scale, but each is liable to irregularities 

 of height at different points, and that this is the cause of the 

 occasionally dotted appearance of the lines for which Dr. 

 Goring blames some microscopes. If the instrument is set 

 so as most distinctly to observe the longitudinal lines, then a 

 slight removal of the object to a greater distance will bring 

 the irregular eminences of each line or rib into view. 



We are now prepared to observe upon the " mysterious '* 

 subject of the diagonal lines. They appear to me to be, as 

 Sir David Brewster states, an optical illusion produced by the 

 arrangement of the cross fibres ; a similar effect, on a larger 

 scale, is often observable in printed fabrics for gowns or waist- 

 coats, in which two sets of diagonal lines are seen at a certain 

 distance, but when the pattern is examined closely they are not 

 there. When the scale is removed to a sufficient distance only 

 for showing the cross fibres and not the longitudinal fibres, 

 and the light in the position of the scale is a little oblique, 

 then this delusive appearance takes place. Dr. Goring ob- 

 serves that it was best seen by a lens of moderate power; and 

 certainly, when the magnifying power is only such as not to 

 magnify too much the divisions between the lines, then the 

 delusion of a continued diagonal line is more complete; and 

 it will always appear most complete towards the serrated end 



