316 THE AMERICAN MONTHLY [Nov 



this latter case the removal of the cover-glass, with its 

 manifest dangers, is not necessary. The podura scale,on 

 account of its variability, is not a trustworthy guide to 

 the testing of lenses in unskilled hands, and we put no 

 faith in the exhibition of the inner markings, on which so 

 much stress is laid. They are easily shown by an indif- 

 ferent lens with a little "stopping down" of the dia- 

 phragm, and we believe them to be caused by an optical 

 effect of diffraction. The true nature of the markings on 

 a Podura scale has, however, not yet been satisfactorily 

 explained]. 



Markings are bolder, less continuous, and, when prop- 

 erly focussed, each shows a more distinct bright line down 

 the middle. The "notes of exclamation" are thus more 

 easy to show up separate and distinct than in the ordina- 

 ry scale. When mounted in balsam the marks become 

 almost, if not quite, invisible. Dr. J. W. Arnold succeed- 

 ed in detaching the "exclamation marks" by means of an 

 electric spark. Lepidocyrtus violaceus, a smaller species 

 than curvicollis, like that insect, is abundant in cellars. 

 I refer, with some doubt, to this species. 



Both that and the previous kind seem to congregate un- 

 der or about a sheet of .paper placed in the cellar, especi- 

 ally if a little flour has been sprinkled on it. I think they 

 like the shelter, and dislike light. The scales are more 

 irregular in shape and the markings finer, but the lines 

 of marks are wider apart and although the marks are 

 more continuous, their heads are rather more bulbous than 

 in L. curvicollis. Beckia argentea is a silvery little insect, 

 far smaller than either of the preceding and much lighter 

 in color, which I found in considerable numbers running 

 about stones on a wall at Colwyn Bay at dusk, when the 

 dew was settling. I could only "bottle" one in a specimen 

 tube, because when I tried to get another the first escaped. 

 The scales are very thin and transparent, the markings 

 very delicate and fine, and the "exclamation marks" in 



