32 



LECTURES TO SCIENCE TEACHERS. 



tine, when in contact with the dichromates viz., non- 

 absorption of water and insolubility where light has acted 

 that a variety of other photographic printing processes are 





Fm. 9. 



founded. On the later reaction is founded the autotype process, 

 where the image is formed absolutely of coloured gelatine, all 

 the parts not acted upon by light having dissolved away ; 

 whilst on the former are built up all those processes which 

 produce prints in graduated tints of greasy ink after an image 

 has been obtained on gelatine that has been hardened and 

 rendered insoluble in water (though still leaving it capable of 

 absorbing water in the parts not acted upon by light), by the 

 addition of such substances as tannin, chrome alum, &c. As 

 examples of such processes I may mention the heliotype, 

 albertype, and autotype mechanical processes. 



I have now come to the end of the time allotted to me, 

 and I trust that the explanations as far as they have gone 

 have been clear ; but in treating of such a large subject as 

 photography, it would be necessary for you to listen to me 

 for as many days as you have hours, in order that I might 

 enter into the details of much which I have merely been 

 able to glance at. 



