88 



HOMING AND RELATED ACTIVITIES OF BIRDS. 



The experiment naturally falls into two parts: first, accurate determination 

 of the limits of the spectrum at the violet and red ends; second, plotting of 

 the sensitivity curve for monochromatic light. At the end the attempt is made 

 to compare these findings with those obtained from similar tests upon man. 



A very large sjjectrometer was used, the ground plan of which is shown in 

 figure 8. It consists of the source L, a Nernst automatic lamp bm-nt at 110 

 volts, D. C. The lamj) was controlled by regulating the amperage (the amme- 

 ter reading to A of an ampere) ; a single achromat condenser gave an image of 

 the source upon the slit S (this slit is accurately controlled by a micrometer 

 screw) ; the collimator Cm, which is a compound 4-inch portrait lens; the dense 

 flint-glass prism P, with 4-inch face compound portrait lens O, used as the ob- 

 jective, and the slit S2, which was mounted upon a carriage controlled by a mi- 

 crometer screw. The slit system was directly calibrated in termsof wave-length, 

 so that any possible region could be immediately admitted to the dark room. 



Fio. 8. — Ground Plan of Double Spectrometer. L, source (Nernist); Cd., condenser; Si, slit; 

 Cm., collimator; P, dense flint glass prism; O, objective; ^2, selecting slit; B. sec, Brod- 

 hun rotating! sector; Cm.j, collimator for auxiliary spectrometer; Pi, direct vision prism; 

 Oi, objective for auxiliary spectrometer; S3, final selecting slit; TR., total reflection 

 prism throwing light on plaster surface. The small sketch at the right is a vertical section 

 through T. R., showing the way the beam is finally projected upon the plaster surface 

 (stimulus patch). 



For the work on limits the Brodhun sector (B. sec.) and the auxiliary spec- 

 trometer shown in the diagram were not used. These were employed in the 

 second jjart of the work — the determination of the relative stimulating effect 

 of the clifferent spectral regions. 



The Brodhun sector is too well known to require detailed description. It 

 consists of a series of rotating prisms which revolves the light around a sta- 

 tionary sector. It permits of the immediate control of the intensity of the 

 light without stopping the motor for the needed adjustment. The sector 

 openings are controlled by an arm extending from the housing of the sector. 

 An arbitrar.y scale in millimeters running from zero, where the sector is fully 

 closed, to 140, where the sector is open at the maximum allowed by its con- 

 struction, permits one to adjust the intensity of the light with great accuracy. 



For reasons which will appear in the text, it was found necessary, in the 

 second part of the work, to secure monochromatic light practically free from 

 admixture with white light. This can not be secured with a simple spec- 

 trometer system. Accordingly, after a given band of light hatl been selected 

 by the slit Sj, it was passed through a second spectrometer, of which Cm,, in 



