so HOMING AND RELATED ACTIVITIES (JF BIKDS. 



Tlie experiment naturally falls into two parts : first, accurate tletermination 

 of the limits of the spectrum at the violet and i-ecl ends; second, plotting of 

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

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



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

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

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

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

 the source upon the slit S (this slit is accurately controlled b.y a micrometer 

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

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

 jective, and the slit S^, 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 I'oom. 



r 



Fig. 8. — Gro>md Plan o( Double Spcctromoter. L, source (Nernst); Cd., condenser; Si, slit; 

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

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

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

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

 through T. R., .showing the w.ay the beam is fin,ally [irojected upon the plaster surface 

 (stinmlus jjatch). 



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

 t IOmeter shown in the diagram were not used. These were employed in the 

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

 of the (liffei'ent spectral regions. 



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

 consists of !i 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 stop])ing the motor for the needed adjustment. The sector 

 oi)enings arc controlled by an arm extending from the housing of the sector. 

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

 closed, to 140, where the sector is open at the maximian 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 foimd necessary, in the 

 second i)art of the work, to secure monochromatic light ]i)-actically free fi-om 

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

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

 by the slit S;, it was i>assed through a second s]iectrometer. r.f v.-hicli Cm,, in 



