78 EVENINGS AT THE MICROSCOPE. 



considering, there is considerable diversity in details. 

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

 projecting 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 

 produced 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 minute 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, kc. 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 



