'B48 VERHOEPF AND BELL. 



error would be in the direction of safety, that is, it would tend to 

 set the Umit of abiotic action at too long a wave length. It will be 

 noted that in this series of screens the transmission grades off with 

 considerable regularity. 



In addition to these screens the following absorbing media were 

 used in some of the experiments. These did not prove of value in 

 determining the limits of abiotic action, but are described here because 

 the experiments in which they were used are important in reference 

 to possible retinal effects, owing to the long exposures given. (Plate 5.) 



Material Limit of Absorption 



8. CuClo 1.5% solution in 5 cm. quartz 



cell 320 ^iju, beyond 700 ^t/x 



9. Blue Uviol glass 2 mm. 285 /x/x and beyond 470 ju^t 



10. Auramine O .001% solution in 5 cm. 



quartz cell 250 nfj., 400 fx/j. to 450 nn 



11. (9+10) 



Auramine O, which we tried in several concentrations, is remarkable 

 for the freedom with which it transmits the extreme ultra violet while 

 absorbing the violet end of the visible spectrum rather strongly. 



For producing intensive exposures, and particularly for work on 

 the retina the magnetite arc here described was reenforced by the 

 use of a quartz lens system. For one set of experiments we employed 

 two piano convex quartz lenses each of 42 mm. diameter and 18 cm. 

 focal length. These two were generally employed placed with the 

 plane faces in contact either with each other or with one of our screens, 

 making in fact a single lens of 9 cm. focal length for parallel rays. 

 This lens was placed 20 cm. from the arc, an image of which was formed 

 14 cm. beyond it, at which point the eye was placed. In this set of 

 experiments in addition to abiotic effects in the cornea and lens, 

 small circumscribed heat effects were obtained in the retina analogous 

 to those of eclipse blindness. These will be discussed later (page 697). 

 In another set of experiments the apparatus was assembled as shown 

 in Figure 4. The lenses referred to, B, were placed at 12 cm. from the 

 arc flame A. In the converging cone of rays produced by B, was 

 placed at a distance of 12 cm. therefrom a double convex lens C of 

 quartz cut across the axis, 23 mm. in diameter and of 14 mm. focal 

 length for parallel rays. In the combination used, C brought the 

 rays to a focus at about 12 mm. from its outer apex at or near which 

 point the cornea of the eye D was located in the experiments. The 



