380 Mr. J. S. Ames on the Concave. Grating 



includes 1 Angstrom miit. For gratings of 10 ft. radius, the 

 scale is diminished in the ratio of 100 : 215 or 20 : 43. 

 ■ Since with a concave grating all the spectra are in focus at 

 the same time, it is important to know what wave-lengths of 

 the different spectra are on the photographic plate or in the 

 field of the eyepiece, for any position they may be in. For 

 this purpose I have given a diagram of the overlapping 

 spectra on the Plate. This explains itself: — Wave-length 

 6000 in the 2nd spectrum coincides with wave-length 4000 in 

 the 3rd spectrum, with wave-length 3000 in the 4th spectrum, 

 and so on. The vertical lines give the range of the different 

 gratings, as explained above. If it is desirable to cut off any 

 interfering spectrum, glass plates or absorbing solutions may 

 be used. A list of the principal absorbents, and the parts of 

 the spectrum which they let through^ is given below. 



Greenish plate glass 3300-8000 



Salicylic acid in alcohol, saturated in quartz cell 3500-8000 

 Aesculin, 1 gr. in 1 oz. water, with one drop of 



ammonia — fresh 4100-8000 



Potassium ferrocyanide 4400-8000 



Primrose or Auihne yellow 5000-8000 



Fluorescine or Chloride of gold 5200-8000 



Chrome alum \ C 3200-3700 



Malachite green f J a 



Bitter-almond green f \ ^"^ 



Brilhant green ) ( 4600-5200 



Cobalt chloride 3400-4500 



Gentian violet, strong 3600-4600 



and 6000-8000 



Potassium permanganate 3900-4600 



and 5800-8000 



For example, using a 10,000 grating and photographing in 

 the 4th spectrum, the following absorbing solutions are used 

 at the places specified : — 



At 

 3800 Cobalt chloride in water. 



4000 Cobalt chloride or Gentian violet in water in glass cell. 

 4200 Potassium permanganate or Gentian violet in water. 

 4400 Aesculin or Potassium permanganate. 

 4600 Aesculin. 



4800 Aesculin and Malachite green in water. 

 5000 Aesculin and Potassium ferrocyanide. 

 5200 Aesculin and Potassium ferrocyanide. 

 5400 Aesculin and Primrose. 



