SPECULA BY HAND, AND EXPERIMENTS WITH FLAT SURFACES, 91 
do not maintain that variations even in the thickness of the 
film, as deducted by me, cannot by polishing be carried so far as 
to cause perceptible alteration in the figure of the speculum; in 
fact, that such can take place will at once be apparent by exam- 
ination of the table for corrections as given in this paper ; but, as 
to the visibility of the scratches proving echo in regar 
thickness of substance in which they appear, seems doubtful. 
Supposing the least particle which can be perceived by natural 
vision more than sslio esd - an nck, it follows that 
whether the thicknes of the film be sy Or zy0000 part of an inch 
the depth of scratches could not be seen * nor = these appear to 
our view, as stated, by the light reflected from their sides, but 
simply by the width of the silver removed by them and the con- 
sequent exposure of the less reflecting surface below. 
ct 
° 
Discussion. 
Mr. H. C. Russett, B.A., F.R.S., &c., stated that he was sure 
that ali the members joined him ia thanking Mr. Madsen for his 
paper, and for showing such a beautiful experiment by means of 
which they could see ‘the thickness of the air space between the 
glasses ; for, by measuring the relative scuttitin of the dark lines 
to iia by Mr. (no ow Dr. , Schroeder, when bs was in Sashes some 
12 years ago, and told how, by placing his finger on to the reflector, 
the heat of it bent it so much that the i image was put entirely out 
of the fieldof view. Mr. Madsen’s experiment was more beautiful, 
but not so easily applied as that of Dr. Schroeder 
Dr. roeder constructed a large refracting telescope for the 
Sydney Observatory, on which he bestowed oreat care, and it is 
supported in a solid brass cell; yet, if a slight pressure is made 
upon the side of the brass cell, the whole lens is bent. Great care 
is required in constructing large glasses, and some specialists re- 
fuse et a any larger than 8 inches in diameter. 
r, he believed that one will shortly be made for the 
Lick Giaecciaty, on Mount Hamilton, 36 inches in diameter. 
[Three diagrams. ] 
