Mr. T. Gaffield on the Action of Sunlight on Glass. 523 



This last experiment proves the propriety of the preference 

 given by photographers to blue glass for skylights, because it 

 transmits the blue rays, which exert the most actinic power. 

 But it may be added, that a colourless white glass, or bluish 

 white (if one which will not change by sunlight to a yellow 

 or rose-colour, owing to the presence of manganese, or any other 

 cause), is especially good, as it will transmit all the rays, and 

 among them the actinic or blue ones. In proportion as any 

 kind changes to a yellow or rose-colour, it will lose its power of 

 transmission and its value as photographic glass. I have seen 

 specimens of the two kinds of white crystal sheet made in Mas- 

 sachusetts, before alluded to, which answered the demands of 

 photographic artists. Of foreign glass, I have noticed a fine 

 bluish-white sheet, made lately without manganese, from a cer- 

 tain excellent manufactory in Belgium, and one kind of English 

 crown glass. 



Should plate glass be required, the most permanently endu- 

 ring, or least likely to assume a yellow colour, are a superior 

 kind of white plate made by the French and Belgian Plate-Glass 

 Companies, and an excellent quality of German crystal plate 

 made at a long-established factory in Hanover. 



I desire to say here, however, that it is not the place where 

 any glass is made which determines its good character, but the 

 actual constituent materials and the superiority of its manu- 

 facture. 



Manufacturers are frequently changing their mixture or 

 "batch;" so that any results given with one set of samples 

 might differ from those made with another set from the same 

 manufacturers. For this reason, in noticing any differences 

 which may occur in experiments made by any of our readers, 

 this fact should be considered as an explaining cause. 



I have seen specimens of glass from a factory which changed 

 to a yellowish tinge in a few months, others which changed to a 

 purplish hue, and still others from the same factory which 

 hardly changed at all. A difference in the mixture (or batch, 

 as it is termed) makes a difference in the tinge of the specimens 

 from the same factory, both before and after exposure to sun- 

 light. The chief points for photographers are to get glass made 

 from as pure materials as possible, of as light a colour as prac- 

 ticable, and free from oxide of manganese. A glass like either 

 of those named above as most easily transmitting the actinic 

 rays might be good for one year or more, and then become very 

 much injured for photographic effects by the change of colour to 

 yellow or pink by sunlight. 



Any photographer can make these observations practical by 

 testing the action of sunlight for six months, or a year, on all 



2 M 2 



