BENTLEY GLASS 905 



loiiiul bcLwccn the spectral sensitivity ol rods and cones and the ab- 

 sorption spectra ol rhodopsin and iociopsin in the trog, snake, cat, 

 and guinea pig. The same holds for porphyropsin and cyanopsin in 

 the tench. The Pinkinje phenomenon — a shift of maximal spectral 

 sensitivity toward the red — can be treated entirely satisfactorily as a 

 shift from rod vision in dim light to cone vision in bright light. 

 Similar observations for the human rhodopsin absorption spectrum 

 and rod spectral sensitivity (VVald and Brown) and for the visual 

 pigment and spectral sensitivity of Liimilus (Graham and Hartline) 

 also exist. As for the visual threshold dining light and dark adapta- 

 tion, Rushton's work showing that the pigment concentration varies 

 inversely as the logarithm of the threshold sensitivity is well supported 

 by recent studies of white rats in Wald's laboratory. Dowling finds 

 that whether the visual threshold is varied by light and dark adapta- 

 tion or by making the rats nightblind through deficiency of vitamin 

 A, a very nearly linear relation between decreasing rhodopsin and 

 increasing log threshold obtains. Wald believes that the relationship 

 may actually be that of a flattened S-curve, approximately linear only 

 in the mid-region; but this eventuality remains to be established. 



Color Vision 



Two contributions on color vision were made to the Symposium, 

 one on spectral discrimination in insects, the other on electrophysio- 

 logical evidence of color vision in the goldfish. Timothy Goldsmith 

 reviewed the former topic. There is evidence of color discrimination 

 in at least six orders of insects, including both primitive types such 

 as the Odonata and Blattaria, and advanced types such as the Diptera, 

 Hymenoptera, Coleoptera, and Lepidoptera. In some cases the evi- 

 dence comes from training experiments in which color discrimination 

 is made a guide to food; in other instances it is electrophysiological; or 

 is based on the optomotor response; and in still other cases it is based 

 on observation of phototaxis. [The optomotor response depends on 

 a reaction elicited from the movement of vertical stripes in the visual 

 field. If the stripes are alternately gray and colored, the hues can 

 be matched so that no response from the insect is forthcoming. But it 

 is found that a pattern of alternating stripes of two colors both of 

 which match a given shade of gray can nevertheless evoke the insect's 

 response. It is consequently concluded that the insect can discriminate 

 these colors.] 



Almost all insects that have been tested show a strong response to 

 ultraviolet. Experiments utilizing phototaxis as a criterion show a 



