176 LUMINOUS AND ACTINIC 



of the spectrum, remains white, whilst every other 

 portion has blackened.* 



Among the many curious instances of natural magic, 

 none are more remarkable than an experiment not long 

 since proposed, by which Daguerreotype pictures may 

 be taken in absolute darkness to the human eye. This 

 is effected in the following manner : A large prismatic 

 spectrum is thrown upon a lens fitted into one side of a 

 dark chamber; and as we know that the actinic power 

 resides in great activity beyond the violet ray, where 

 there is no light, the only rays which we allow to pass 

 the lens into the chamber are those which are extra- 

 spectral and non-luminous. These are directed upon, 

 any white object, and from that object radiated upon a 

 highly sensitive plate in a camera obscura. Thus a 

 copy of the subject will be obtained by the agency of 

 radiations which produce no sensible effect upon the 

 optic nerve. This experiment is the converse of those 

 which show us that we 'may illuminate any object with 

 the strongest sun-light which has passed through 

 yellow glass, the yellow solution of sulphuret of calcium, 

 or of the bichromate of potash these being non- 

 transparent to the chemical rays and yet fail to 

 secure any Daguerreotype copy of it, even upon 

 the most exquisitely sensitive plate. Indeed, the 

 image of the sun itself, when setting through an 

 atmosphere which reduces its light to a red or rich 

 yellow colour, not only produces no chemical change, 

 but protects an iodized* plate from it ; and whilst every 

 other part of the tablet gives a picture of surrounding 

 objects in the ordinary character, the bright sun itself 

 is represented by a spot upon which no change has 



* Attention has been directed to the protecting action of certain 

 rays of the spectrum by Sir John Herschel and others. See the 

 Eighteenth Report of the British Association for an experiment 

 by the Author, "in which it was proved that all the LIGHT rays pro- 

 tected photographic papers from chemical change, and, therefore, 

 convincingly show that light and actinism were not similar powers.. 



