Absorption of Light by Optical Glass and Calc-spar. 381 



Table I. 



Selective transmission of light by a lens. 



Transparency for each wave-length 

 Wave-lengths. in terms of that of the region of the 



D line. 



\=-7530j* 1-059 



•6685 1-046 



•6080 1-015 



•5570 0-964 



•5185 0-906 



•4920 0-867 



•4685 0-826 



•4500 0-812 



•4340 0-777 



•4250 0-750 



Table II. 

 Selective transmission of light by a pair of Nicol's prisms. 



Transparency for each wave-length 

 Wave-lengths. in terms of that of the region of the 



D line. 



\.= -7530i* 1-006 



•6685 1-003 



•6080 1-001 



•5570 0-995 



•5185 0-977 



•4920 0-913 



•4685 0*844 



•4500 0-736 



•4340 0-617 



•4250 0-500 



The lens referred to in Table I. is made of ordinary white 

 crown glass (refractive index 1*549). It consists of two 

 simple plano-convex lenses, the added mean thickness of the 

 two being about 2 cm. 



The Nicol's prisms were of the usual form, their thickness, 

 measured in the direction of the path of the ray, being about 

 50 mm. 



It will be seen that the glass of which the lens was 

 constructed although not more strongly coloured than most 

 optical glass, the tint being quite unnoticeable to the unaided 

 eye, at least when seen through in the direction of the optical 

 axis, is far from being colourless. The selective absorption 

 begins to show itself in the red and the transparency falls off 



Phil. Mag. S. 5. Vol. 33. No. 203. April 1892. 2 D 



