+ 
J. M. Blake—Notes on Diffraction Gratings. 33 
contain remarkably handsome crystals of fieldspar, often of the 
orm of the Carlsbad twins. 
The tellurium ores of Gold Hill occur in connection with 
one of these dikes. See the section on page 26. This di 
varies from forty-five to thirty-five feet in width, trends about 
N. 30° E., and dips approximately 80° to the northwest. On the 
east side is the Red Cloud, on the west, the Cold Spring mines. 
The former, upon a casual examination, showed a well defined 
hanging wall or that on the side of the porphyry, a vein 
quartzose gangue of the vein; with it there is some Zine 
blende. I had no opportunity to examine into any paragenetic 
or other feature of the tellurium ores. 
Art. V.—Notes on Diffraction Gratings ; by JOHN M. BLAKE. 
‘sent for the purpose a ruling on glass 6480 lines to the inch, 
by erfurd 
Mr. L. M. Ruth ‘ Bas 
In reproducing these lines by photography, it is necessary to 
employ contact printing; for no lens would give the required 
definition over so large a surface, either for copying a ruling, 
or for originating a grating, by reducing from a large drawing 
on paper. A ruling one inch square, containing 6480 lines, 
would be represented by a drawing 27 feet square, in which 
the lines would have a separation of one-twentieth of an inch. 
The sensitive plates used in these trials had a hard and — 
perfect albumen surface, which admitted of close contact with 
the ruled plate. The impression was made by a beam of sun- 
light. The resulting photographs were negatives, having 
white lines on a dark ground; the dark spaces being from 
once to twice the width of the lines. They gave more brilliant 
Spectra than the original ruling, and this was doubtless o 
to the greater intensity and contrast of the lines. But it was 
found, on examination, that the photographs had an unexpected 
_ defect; and in investigating the cause of this some interesting 
phenomena were noticed 
_ Am. Jour. Sc.—Turrp Serres, Vou. VII, No. 43.—Juxy, 1874. 
: 3 get 
