164 



BIOLOGICAL EFFECTS OF RADIATION 



basis for quantitative experimentation. They are intended only as a 

 convenience to aid in the selection of suitable equipment. A convenient 

 group of liquid and glass filters is offered by Wood (68). Note especially 

 the long-wave cut-off filter No. 2, Fig. 13. 



A third type of filter, the Christiansen filter, has been subject to a 

 limited use. It consists of a finely divided transparent solid, immersed in 



2,000 



1,000 



4000 



5000 



^000 iqooo 11,000 i2,oooa 



6,000 /;000 8,000 



Vave- Lengfh 



Fig. 12. — Monochromatic filters: 1. Red purple ultra-violet transmitting, 5.89 mm. 

 (thickness). 2. Heat resisting red 247%, 3.49 mm. Light shade blue green, 3.11 mm., 

 Code 428. 3. Heat resisting red shade yellow, 3.38 mm.. Code 348. Dark shade blue 

 green, 3.70 mm.. Code 430. 4. Heat resisting yellow shade yellow, 2.66 mm., Code 351. 

 Didymium, 5.05 mm.. Code 512. Dark shade blue green, 3.59 mm., Code 430. 5. Noviol 

 A, 3.06 mm., Code 0.38. Violet, 3.25 mm.. Code 511. 6. Noviol O, 2.15 mm.. Code 306. 

 Red purple ultra, 6.00 mm.. Code 597. Light shade blue green, 3.98 mm.. Code 428. 

 7. Light blue nultra, 1.50 mm.. Code 534. Red ultra, 4.04 mm., Code 584. Light shade 

 blue green, 3.91 mm., Code 428. 8. Red ultra, 8.87 mm.. Code 584. 



a liquid whose dispersion is much greater, but whose index of refraction 

 coincides with that of the solid at some wave-length. That particular 

 wave-length will be transmitted without deviation if the cell is made with 

 parallel windows. All other wave-lengths will be scattered. If such a 

 filter is placed in an optical system, with a limited source, preferably 

 circular, and a restricting diaphragm placed at the image of the source, 

 most of the scattered light will be lost and a fairly monochromatic beam 



