124 Object-glasses and their Definition. [Monthls Microscopal 
II.—Object-glasses and their Definition. 
By F. H. Wenuaw, Vice-President R.M.S. 
Dr. Mappox has obtained photographs of the Podura (Degeeria 
domestica), by means of a Wales’ 1th, showing a beaded appearance, 
similar to that observed by myself, and has kindly offered to leave 
the negatives with some one who will supply prints to those de- 
sirous of possessing them, and who have not succeeded in develop- 
ing such a structure. 
The appearance is obtained at the entire sacrifice of the “ note 
of admiration” markings, and that distinctness and sharpness of de- 
finition that so delights the eye of the optician, and resembles an 
irregular ribbing, as if a painting had been made on a panel and 
the graining comb passed transversely in a slightly undulating 
course across the ribs, and it is remarkable that where the cross 
lining has commenced with a larger nodule, this increased size has 
a tendency to continue right across the scale. 
I have recently made some careful examinations of different 
species of Podura, and cannot confirm Dr. Pigott’s statement that 
the “ note of admiration” markings are caused by the crossings of 
a series of oblique ribs on the upper and under sides of the scale. I 
find that the best illumination is sunlight: without sufficient reason 
this is generally repudiated. In cases where the colour correction 
of the object-glass is imperfect, the chromatic errors are so developed, 
that accurate observation is difficult; but with a glass correctly 
achromatic, sunlight is really a valuable aid to investigation. 
I have never seen the varieties of Podwra so finely shown as by 
this means. I use no stop under the stage, but throw sunlight on- 
the scale in a direction from the top, towards the root, at an angle 
of about 30° with the concave mirror. The characteristic markings 
of the scales are thus splendidly shown by the ='5th of a blue colour 
on a pale-green ground. The highest eye-piece may be used, and 
the tube drawn out to its utmost length, without sacrifice to distinct 
vision, thus obtaining a magnifying power of near 11,000-diameters. 
“The note of admiration” markings are only seen in focus, and then 
show scarcely the vestige of a beaded structure. By focussing away 
the markings fuse together lengthwise, and appear like irregular 
knotted cords—the so-termed “ beads.” The outline sketch (p. 125) 
represents an oblique view taken by the camera lucida of the 
markings on Podwra, magnified about 6000 diameters. The margin 
of the scale (mounted on the covering glass) had curled away so 
as to bring several of the ribs gradually into profile. 
Up to the present time I have been unable to find any of the 
proper insects in my locality in order to fulfil my intention of torre- 
fying the scales as they lay on the glass, for the purpose of partly 
