''run!:Ii.Trrff ] Markings on the Podura Scale. 13 



III. — Tlie Marhmgs on the Podura Scale, being a Postscript to 

 a Paper on High Power Definition. By G. Koyston Pigott, 

 M.D. 



Mr. Keade having kindly furnished me with some shdes* of 

 Podura Scales, viz. Mr. M'Intyre's specimens of " Inner Surface of 

 Pegeria Domestica and Macrotoma Major" I find that the upper 

 beading is so coarse that it may be plainly made out with a good 

 half-inch objective, a C eye piece and 16 inches of tube. At Mr. 

 Eeade's request I exhibited the beading to him very rapidly (very 

 little adjustment being required) with the half-inch. 



They may be seen also on a dark field, by employing oblique 

 light from the ordinary concave mirror, at a greater obliquity than 

 the semi- aperture of the half-inch objective. I now see them. 

 The axis of the scale is inclined at about 30 degrees to the 

 direction of the light. By this light they appear yellowish green. 

 I see no under-beads. 



Fitting the hydro-objective to the one-eighth Powell and 

 Lealand, 1 shall expect a gorgeous field. 



A lady, who is now viewing the scale, says " the beads look 

 just like rows of peas in the pod." But the water is now insinuating 

 itself, and bright spots four times the size of the beads arise hke 

 blebs of a hght green colour. One rouleau vanished instantly. 



The "green peas" are mingled in alternate rows with the 

 upper deep orange-red beads : and each set can be seen with black 

 crescentic shadows. The water advances and gradually obliterates 

 the vivid and lovely prismatic colourings. 



But to me, moie striking than all those appearances are our 

 old friends the black markings of the Podura reappearing alongside 

 of our newer acquaintances, the upper and lower beads. At the 

 same time the lattice-work of rows of beads is extremely beautiful. 



I now examine the other Podura slide from insects bred by 

 Mr. M'Intyre. Same arrangement without condenser. 



The Macrotoma Major. — The beads now appear in parallel 

 rows ; orange-red above, and between them and below them appear 

 rows of intensely blue beads. They all gradually diminish as they 

 go towards the quill end of the scale, appearing beautifully less. 



They are very easily seen in any position of the scale with 

 ordinary oblique mirror illumination. 



These scales come most opportunely, for they render that easy 

 which is exceedingly difficult in the regular test scale of my plate. 



* It is very noteworthy that both the upper and under sides of these seales 

 appear identical in their physical phenomena— structure and colour. Mr. Mclntyre 

 informs me the insect is here pressed upon the slide, instead of upon the cover, to 

 show the under surface upwards. 



