52 PROCEEDINGS OF SOCIETIES. Se ee 
too windy to obtain sharp pictures, causing considerable vibration to 
the large mirror. 
I will spare you a recital of the many trials I have had, and the 
difficulty to secure anything like the said beaded structure; and re- 
member I have not attempted to obtain what is ordinarily supposed to 
be the optician’s appearance ;—just the reverse, everything else except 
the right ones you'll say,—so be it. 
Now the appearances recorded are most fleeting, and required the 
hand to be on the pinion of the horizontal and vertical adjustment of 
the mirror to the moment of putting the sensitized plate in its place. 
Unfortunately I have had sent me some miserable collodion, which 
would not intensify after the usual methods, so that to procure printing 
density I had to resort to all sorts of expedients, and on so doing 
secured much obscurity. 
I don’t think we are yet in order as regards the true structure of 
this scale, and fancy analogy from other scales should enter into the 
debate, for to describe the true structure of a transparent object by 
transmitted light is most difficult: not that I am going to open the 
question by aflirmation; this I leave for wiser heads, with better 
appliances. 
However, to start from a point, and as I shall not be hanged for 
thinking, I may say that the testing of the scale under very varied 
illumination, and my never having been satisfied with the appearances 
obtained by myself photographically, I fancy the following may meet 
my undecided views. 
I take the scale to be a truly ribbed structure, finely beaded on the 
ribs, which deviate or undulate, approach to or widen from one 
another; when these undulations are focussed so as to give the 
approximating parts of the upper and under surfaces, you have the 
undulation of the ribs, converted into notes of admiration and the 
beading lost, except under most favourable states of illumination and 
perfection of the objective corrections. The undulations give the wavi- 
ness, and by slight obliquity in the illumination are thrown from the 
ribs farther, so that diffraction comes fairly into play, and may produce 
anything according as the pencils traverse the scale at various angles 
to the undulating ribs. The prominent round beads shown at some 
parts of the focus at the head of the notes of admiration, may then be 
due to the union of the ribs at the points where the widening springs 
from the same, as is seen so continually in photographing objects with 
ae, large hemispherical bosses, at the bases of the 
| apparent or made to appear hexagonal areas, 
when they touch each other, which is shown 
beautifully in a photograph of Coscinodiscus taken 
with Mr. Wenham’s 1; long since. See Fig. 2. 
Now whether this ribbing should be con- 
sidered as a true rib set at right angles to the 
plane of the intervening structure, and that 
intervening structure a single membrane, or 
whether the membrane be double and the rib seated on its surfaces, 
one on either side, or whether the ribs are exactly opposite one another, 
