29 



alcohol has not been added, the extract must be applied to the 

 paper immediately. Most of the papers so prepared require 

 an exposure of many days, from twenty to thirty, to produce a 

 decided effect, and the pictures obtained are not always permanent. 

 This will of course preclude their being of practical utility ; but 

 the changes produced are so remarkable, that we could not, with 

 propriety, omit mentioning them. A full account of Sir John 

 Herschel's experiments will be found in his Memoir, or "The 

 Action of the Rays of the Solar Spectrum on Vegetable Colours, 1 ' 

 etc. published in the second part of the Philosophical Transac- 

 tions for 1842. 



Similar effects are produced by light in the gums resins and residua 

 of essential oils, when thin films are spread upon paper or on metal 

 plates. A paper prepared with an alcoholic solution of guaiacnm, 

 and placed in an aqueous solution of chlorine, acquires a beautiful 

 blue colour ; it is very sensitive, and may be used for copying 

 engravings, the resulting picture penetrating the paper, and 

 appearing on the back with almost the same intensity as on the 

 face. The images, however, speedily fade. 



In the preceding pages we have endeavoured to include all the 

 Photographic processes which will be really useful to amateurs. 

 There are many varieties of all these; every successful practi- 

 tioner having his favourite formula, or modus operandi. To record 

 all those that have been announced to the world, during the last 

 two or three years, would require a volume, and would confuse 

 rather than direct. We would recommend our readers to acquire 

 a practical acquaintance with such as have been described ; and 

 then, if they have some chemical knowledge, a small portion of 

 time devoted to the consideration of the general principles upon 

 which they are all conducted, will possibly enable them to intro- 

 duce divers modifications and improvements. We have already 

 pointed the way to such enquiries, in referring to Sir John 

 Herschel'si papers in the Philosophical Transactions, and to Mr. 

 Robert Hunt's Researches on Light, which, with a few papers 

 scattered through some of our scientific periodicals, comprise 

 everything of importance that has been written on the subject. 



