EYE-PRESERVING GLASS FOR SPECTACLES. 
7 
0'07 mm. thick entirely cuts off the luminous rays, and even the sun’s disc is only just 
seen through a Hake 0‘06 mm. thick. 
In addition to specimens from Norway, Messrs. Attwater and Sons sent me black 
biotite from German South-East Africa, and some fine pieces of “ black amber mica 
from Africa. The African mica is uniform in colour, and easily splits into flakes of 
great regularity. The German mica is difficult to split into uniformly regular 
flakes, and therefore varies considerably in colour. 
Messrs. F. Wiggins and Sons allowed me to select some large sheets of dark 
brown and black biotite from their stock. The sheets when split are uniform in colour, 
and in thicknesses below about 0'30 mm. are sufficiently transparent to allow the eye 
to detect a Nernst glower. Mica differs, however, in transparency, one flake from a 
sheet being opaque at 0‘24 mm., while a flake from another part of the same sheet is 
slightly transparent in a thickness of 0'34 mm. I also received good black mica from 
Mr. Henson, who gave as its locality Iienfrew, Canada. 
Many experiments have been carried on with all these kinds of brown mica to 
find a quality which would cut off the rays which at Kew were called the “ scorching 
rays ” (the infra-red rays), and some of the best results were obtained with the 
Norwegian mica from Messrs. Attwater and Sons, and the black amber African mica 
from Messrs. Wiggins and Sons. 
There is a certain gradation of transmitted rays according to the thickness of 
the dark brown micas. All of them cut off rays at the blue end of the spectrum, 
and as the thickness increases the portion of the spectrum obstructed rapidly tends 
towards the red end, until a mica is found which affects the photographic plate in a 
narrowed band round the line B, the exposure being from ten to twenty times as long 
as would be required for this part of the spectrum to impress itself, were no mica to 
intervene. To increase the thickness of the dark mica soon obstructs all rays in the 
red visible to the eye. 
Examined by the thermometric apparatus, a thermopile, and in the radiometer 
balance—described below—the dark micas which allow any trace of visible rays to 
pass are strongly diathermic. As the thickness of mica increases the deflection of the 
index spot of light gets less and less, until there is very little action at all. Judging 
from analogy it is most probable that as the thickness increases the heat rays are cut 
off in regular gradation from the line B in the red to the longest rays of heat which 
will affect the radiometer balance. 
This is only a hypothesis and does not take into account the possibility of there 
being dark bands in the infra-red portion of the heat spectrum. Still the hypothesis 
as a working tool has been of considerable use, and has helped me to select dark mica 
obscuring media—giving good and concordant results. 
Were I to take at random a piece of black mica which would cut off all the rays 
visible to the eye, and not allow any red and ultra-red rays to pass that would affect 
the panchromatic plate, I should have no certainty that the piece of mica would not 
