124. PROCEEDINGS OF SOCIETIES. 



The best test-scale is that in which the raarkingrs are 

 ■wedge-shaped, very black and brilliant, but with a narrow 

 light space towards the top of each ; they are consecutive in 

 the longitudinal direction of the scale, and no transverse 

 lines whatever are present; the stronger the markings the 

 better, as the test consists, not in the mere fact of separating 

 the dots (a result easily obtained with an inch objectg-lass), 

 but in the way in which the markings are shown ; this point 

 also requires illustrations, but, without any drawings, the fol- 

 lowing advantages of the object may be mentioned : — A 

 proper scale will show most distinctly the condition of the 

 chromatic and the spherical aberrations, the latter more es- 

 pecially being evident, by the care that a good object-glass of 

 highest power requires, in its adjustment for the thickness of 

 the covering glass ; and when this is correct the appearances 

 presented immediately either within or beyond the focus 

 should be exactly the same. Bad workmanship of any kind is 

 also strikingly evident. 



The habitat of these insects has hitherto been described as 

 damp cellars, whereas the drier the place in which this species 

 can be found at all, the more likely are they to be furnished 

 with superior scales. 



The best way to remove the scales, is to press a piece of 

 glass very lightly on the live insect, taking care to select those 

 that are in the best condition. 



The author's opinion as to structure was entirely con- 

 firmatory of that already quoted from the ' Micrographic 

 Dictionary ;' and proofs were given of the darkness of the 

 markings being due to the refraction or reflection of light 

 (provided no pigment be present), by an analysis of the ap- 

 pearance of the scale of Lepisma saccharina, when moisture 

 was present on either surface. 



Especial attention was also directed to the remarkable 

 appearances produced by the alterations in the direction of 

 the light, more especially when the markings of two surfaces 

 were superposed, and crossed each other at different angles ; 

 so that under many circumstances it becomes impossible 

 to determine the structure of a transparent object from its 

 appearance only, whether it be illuminated from above or 

 below. 



These remarks do not apply to the Podura scale, for only 

 one, and that the outer surface, has structure upon it. Its 

 illumination under a power of 1300 linear, from above, is 

 easily effected by the ordinary side condensers, provided the 

 object be uncovered, as, with the oblique illumination that 



