612 BIOLOGICAL EFFECTS OF RADIATION 



that Bertholf's method is consequently inadequate. It may also be said 

 that the difference in the stimulating efficiency of two lights is not propor- 

 tional to the frequency of selection and that Sander's method is conse- 

 quently also inadequate. The experiments should be repeated with 

 Bertholf's method, but with several series of monochromatic lights equal 

 in energy in each series but different in different series. 



Hess (103, 104) maintains that bees and other insects distinguish 

 different regions in the spectrum, owing to difference in brightness or 

 brilliance, i.e., that insects do not have color vision. Lubbock (129), 

 Frisch (74), Kuhn and Pohl (122), Kuhn (119), Kiihn and Fraenkel (121), 

 and Bertholf (14) conclude that bees can distinguish between colors or 

 chromas which are for them equal in brightness or brilliance. Frisch 

 contends that only two chromas are distinguished by bees, blue-violet 

 and green-yellow-red. Kiihn and Pohl maintain that four chromas are 

 distinguished, yellow-green, blue-green, blue-violet and ultra-violet. 

 Kiihn and Fraenkel conclude that bees can distinguish eight chromas. 

 Bertholf maintains that they can distinguish between the following pairs 

 of chromas when they are equal in brilliance: blue and violet, blue and 

 green, blue and yellow, blue and red, green and red, green and yellow. 



The training method was used by all these investigators and the 

 results obtained by all clearly show that the bees distinguished between 

 the colors used, but in some of the experiments the possibihty of selection 

 on the basis of differences in brightness and in other characteristics was 

 not excluded, especially brightness due to reflection and radiation of ultra- 

 violet (Lutz 134, 135). Bertholf appears, however, to have excluded 

 this possibility. Color vision appears consequently to be definitely 

 established for bees. But so far recognition of only five to eight colors 

 has been demonstrated for them; whereas man, according to Nutting 

 (180), can distinguish 130 to 180. Color vision is therefore probably 

 much less acute in bees than it is in man. 



d. Visual Acuity. — From results obtained with the training method 

 Baumgartner (11), using yellow and blue figures, concludes that bees can 

 barely distinguish squares 5 mm. wide at a distance of 2.5 cm. and squares 

 10 mm. wide at a distance of 10 cm., but he thinks much smaller spots on 

 flowers are recognized. Results obtained by him in a study of the struc- 

 ture of the eye support these conclusions and indicate that visual acuity 

 is highest in the central, anterior portion of the eye and decreases rapidly 

 dorsally and ventrally but only slightly posteriorly. 



It is well known that movement of a surface containing alternate light 

 and dark areas induces compensatory movements in insects (Rddl, 189; 

 Buddenbrock, 22; et at.). These movements are doubtless types of 

 reflex responses to rapid changes in luminous intensity on the retina of 

 the eye. Schheper (196) made use of this fact and tested the response 

 of a number of different insects to movement of a background containing 



