156 Intelligence and Miscellaneous Articles. 



method was applied to the larger apparatus I got further into 

 the ultra red than I had done before ; and such a plate you see 

 before you : the wave-length of the last line of my group of lines 

 visible on the plate is about 10,300 tenth metres. Erom indica- 

 tions on other plates I am inclined to think that we may get as 

 far as 12,000 tenth metres, w T hich it will be admitted is a tolerable 

 distance to travel along the invisible spectrum. In these ultra 

 regions the lines are faint, but perfectly measurable under a 

 moderate magnif ying-power ; and I have therefore proposed to 

 myself to make a map of the ultra red. For the purpose I pro- 

 pose to use the overlapping of the higher order of the spectrum 

 over that used. By cutting off the red in one case and using half 

 the length of the slit for one exposure, and then by cutting off 

 the blue and using the remaining half of the slit for the other, 

 we shall have one spectrum over the other. The wave-lengths 

 of the most-refrangible rays are known ; and since the dis- 

 persion of the higher order is double that of the one below it, the 

 wave-lengths of the latter can be accurately ascertained. When 

 once a scale is obtained, the greatest difficulty will have 

 vanished. 



Now as to the process. My object has been to weight the 

 molecules of silver bromide that they may absorb the red rays. 

 With ordinary silver bromide the film allows these very rays 

 to pass through, whilst a blue absorption takes place. In other 

 words, my endeavour has been to find a heavier molecule of a 

 sensitive salt, which shall answer to the swing of the waves of 

 the red and ultra-red rays. This I first accomplished (as I stated 

 at the time) by adding resins to the silver salt and forming what 

 I may call a bromo-resinate of silver. But I am happy to say 

 that I have secured the same end by, I believe, doubling the 

 molecule of the silver bromide. Now this doubling the molecule 

 is a matter of manipulation more than of chemical knowldge : 

 and I might describe the process in detail, as I have already done 

 in papers I have published, and yet the double molecule would 

 not be obtained unless careful manipulation were attended to — 

 manipulation easy to follow when seen, but difficult to follow 

 from any description. I should therefore prefer to teach prac- 

 tically any one who is acquainted with silver-bromide-emulsion- 

 making, rather than allow him to be misled by what must be 

 imperfect directions. 



I exhibit two films, both prepared with sensitive emulsion 

 which is composed of exactly similar ingredients, viz. pure silver 

 bromide. You will at once note the difference in colour of the 

 light transmitted. The one which is sensitive to the red and ultra 

 red is of blue tint ; the other is orange. You will see that the 

 blue tint would appear to be due to a physical arrangement of the 

 molecules ; for if a part of the film be rubbed you will see that it 

 changes to a ruddy tint, passing through an emerald-green stage. 



