PROCEEDINGS OF SOCIETIES. 115 
tor, and said that he was inclined to suppose that, at all events, 
for some purposes, the silver reflector would be best. In 
Messrs. Powell and Lealand’s plan a piece of glass of notice- 
able thickness, perhaps a sixteenth of an inch, occupies the 
place of this thin disc of Mr. Beck’s. With as high a power 
as 3th, this sloping piece of glass, not used as an illuminator, 
but simply looked through, allows the Podura scale to be seen so 
as to exhibit those marks that are so beautifully shown in Mr. 
Beck’s drawings. Thus, it does not introduce any noticeable 
errors, although it makes a slight difference in the adjustment. 
With respect to the comparative advantage of a noticeably thick 
glass and a thin one, my opinion is rather in favour of the thin 
glass, except that it is so easily broken. Mr. Beck, however, used 
a size of disc that is very common amongst covering glasses, and, 
when broken, it is easily replaced. With reference to the 
diaphragm which Messrs. Powell and Lealand have copied from 
the American pattern, it will be found useful in cutting off the glare 
from parts of the field not absolutely in focus. Messrs. Powell 
and Lealand fix their piece of glass, which acts as a reflector; but 
Mr. Beck makesit movable. I believe the movable arrangement 
exists in Mr. Smith’s instrument. The power of motion is a 
decided advantage, and therefore I cannot say that I am entirely 
satisfied with their illuminators, as it 1s exceedingly difficult, with 
some objects, to get rid of a kind of milk-and-watery appearance 
of the field. This milkiness sometimes accompanies very good 
definitions of minute objects; and I suggested to Messrs. Powell 
and Lealand that, possibly, introducing a condensor, to get a very 
small pencil of light, might obviate it. Mr. Smith, of Bow, showed 
me an illuminator he had fitted up with a small condensor, and 
whether from this cause or from contracting the aperture, his in- 
strument was more free from milkiness. Professor Smith seemed 
» to think that by his arrangement he gets a more slanting illumi- 
nation; and if so, it would have a decided advantage, for most 
purposes. Diamond-beetle scales come out beautifully with Messrs. 
Powell and Lealand’s or Mr. Beck’s illuminator. In addition to 
a number of vertical lines, some curiously arranged groups of 
curved lines will be seen. I tried to trace a relation between the 
character of the patterns formed by these lines and the colours 
exhibited, but did not succeed. Great importance is to be attached 
to a remark of Mr. Beck, “that you want a means of slightly 
changing the angle of the object when under view.” It must be 
remembered that objects are seen with these illuminators under 
nearly verticalillumination. The effects of this mode of illumi- 
nation may be advantageously studied with lower powers and Mr. 
Beck’s Sorby illuminator. With this instrument a brilliant slanting 
illumination may be instantly changed for a vertical one; and it 
is most instructive to witness the great alteration that takes place 
in the appearance of the object. 
Mr. Broox.—I have not yet had the opportunity of using this 
instrument, and therefore I cannot speak of it from my own expe- 
