and of Indigo Light on Plants. 3 



by a heliostat placed outside my window, along a square tube 

 of wood passing through the shutter. The inner extremity 

 of the tube was closed, and contained near its end a flint glass 

 equilateral prism, one inch on the side and six inches in length, 

 with the axis adjusted perpendicularly. The dispersed light 

 passed into the chamber through an aperture in the side of 

 the tube. All that portion of the beam which exceeded the 

 breadth of the prism was cut off by a diaphragm. The ob- 

 ject of these arrangements was to render the room perfectly 

 dark. The experiments were performed in Virginia, lat. 

 37° 10' N., and continued from July 6 to October 1, during a 

 season of unusual brilliancy and temperature. 



8. Arrangements for the experiments. — Seedlings of tnrneps, 

 radish, mustard, peas, several varieties of beans, and the fol- 

 lowing transplanted specimens were used : — Solatium nigrum 

 el virginianum, Plantago major et minor, Polygonum hydro- 

 piper, Chenopodium rubrum, Rumex obtusifolius. They were 

 placed in boxes with partitions, or planted in jars, and grew in 

 darkness until ready for experiment, -so that they acquired a 

 yellow colour. The number of plants exposed to each ray 

 averaged one hundred, when the smaller seeds were used, and 

 the result indicated was obtained by a comparison of the whole. 

 The age of seedlings is a matter of moment; those which are 

 young and from 1 to If inch, in the case of turneps, were most 

 sensitive ; indeed, these plants were found to give the best re- 

 sults, and used almost exclusively after the first month. The 

 spectrum was allowed to fall on the specimens at fifteen feet 

 from the prism, and undecomposed light closed out by screens. 

 Each ray acted in a separate compartment. 



9. The following extract of one experiment will show some 

 further details : — 



August 13th. — Five jars, A, B, C, D, E, containing each 

 about one hundred turnep-seedlings, were placed respectively 

 in the orange, yellow, blue, indigo and violet rays, at 9 a.m. 

 Day, bright; temperature in shade at noon 80° Fahr., in the 

 sun 95°. Duration of sunshine 6| hours. Result at 3^ p.m. 

 Height of plants at nine o'clock a.m. third column of table. 



Jar. 



A 

 B 

 C 

 D 

 E 



Light. 



Orange. 



Yellow. 



Blue. 



Indigo. 



Violet. 



Height at 9- 



1 inch 

 1 ... 



H ... 

 l ... 

 H ... 



Effect. 



Good green. 

 Full green. 

 Slight olive. 

 Yellow. 

 Yellow. 



Order. 



* The fifth column contains a comparative estimate of the depth of 

 colour, assuming unity as the highest value on this scale, the plants in blue 



B2 



