CH. ll] PHOTOGRAPHIC METHOD. 25 



(30) Gardiner's experiment*. 



A plant growing in a flower-pot (for convenience of 

 moving) is placed in the dark for 24 hours, or until the 

 leaves are found to be free from starch. One of the 

 leaves is now covered with a photographic negative and 

 left exposed to bright light out of doors, or in a green- 

 house, until the evening, when the leaf is tested for starch. 

 It will be found that an accurate copy of the photograph 

 has been printed in starch. 



(31) Effect of rays of different refrangibility. 



The effect of the different parts of the spectrum may 

 be demonstrated by a method similar to that described in 

 Exp. 30, as has been done by Timiriazeff 2 . In the ab- 

 sence of the necessary apparatus we may compare the 

 effects of light transmitted through coloured fluids. Fill 

 a couple of double- walled bell-jars, (1) with potassium 

 bichromate solution, (2) with ammoniacal CuSO 4 solution. 

 Under each bell place a young Tropceolum or clover plant 

 in a small pot, or a seedling plant of any kind dug up and 

 placed with its roots in a bottle of water. The bell-jars 

 should stand in saucers of dry earth or sawdust, so as to 

 insure the exclusion of colourless light. They must be 

 exposed to diffused light in sunshine the temperatures 

 are not the same in the two bell-jars. The exposure 

 should be for 1^- or 2 days. The plants in the blue light 

 will be almost starchless. 



1 W. Gardiner, Annals of Botany, iv. p. 163. 



2 Timiriazeff, Comptes rendus, T. ex. p. 1346. 



