78 EVENINGS AT THE MICROSCOPE. 



considering, there is considerable diversity in details. The 

 form is usually ovate or shovel-like ; the foot-stalk, pro- 

 jecting at a similar angle, is not set on the inferior surface, 

 but in the bottom of a deep narrow incision ; and the ribs 

 are invariably divergent like those of a fan. In these, 

 however, there is a peculiarity of arrangement, which I 

 have never seen noticed, but which is obvious enough in 

 the specimens before us. The ribs on the two surfaces 

 diverge at different angles, those of the upper surface 

 being the more divergent, standing out widely from the 

 foot-stalk, while those of the lower membrane are coarser, 

 and much more nearly parallel, their bases ranging along 

 the hind edge of the scale. The effect of the intersection 

 of the sets of lines at so acute an angle, is to convey the 

 optical impression that the scale is covered with short 

 irregular dashes. 



Such is the arrangement on these scales, which I pre- 

 pared myself from the common Lepisma; but I have a 

 slide marked " Lepisma," from one of the dealers in 

 microscopic objects, in which the ribs on the two sides 

 concur ; but, on one side, there are obliquely divergent 

 lines visible only near the margin, which appear to be pro- 

 duced by wrinkles of the membrane analogous to the 

 transverse dashes on those of Machilis. 



Scales much more delicate than either of these sorts are 

 found on the Podura, a min ute insect, of which there are 

 several species ; which leap, jerking out the bristles of the 

 tail, that are ordinarily carried under the body, like a 

 coiled spring. They are common in cellars, in hot-bed 

 frames, on dunghills, on the surface of water in road-ruts, 

 &c. On the slide before you are some of the smaller 

 scales from one of these insects ; they are exceedingly 

 delicate, and the clearness with which you discern the 

 character of their markings, is a proof of the good defini- 

 tion of your microscope ; this is what is called a " test- 

 object." At first sight they seem covered with ribs like 



