32 



I Feb. 21, 



Eemarks of Prof. Robb, of Trinity College, Hartford: 



We are all indebted to Prof. Goodspeed for a very interesting paper and 

 must congratulate him. There is certainly a great deal of interest in 

 those slides. The first thing that attracted our attention in Hartford 

 about our dry plates was the fact that on a great many of them we 

 noticed second images which were clearly defined, but fainter than the 

 first, having decidedly the appearance of the ordinary halo images of 

 ordinary photography. At first glance one might thick that was due to 

 reflection. I am sure it was not due to any movement that occurred in 

 the plate ; and I am sure it was not due to a violet region of photograph. I 

 think exposures in bright light are a very dangerous thing. It is very 

 possible to get shadow photographs through any of the commercial plate 

 colors ; but in a great deal of our work where we have worked in the 

 ordinary light we have taken the precaution of using an aluminum cover 

 of over a thirty-second of an inch ; and we get second images to the same 

 extent using the aluminum cover. Of course there are various explana- 

 tions of it. It might be from fluorescence or other things that may sug- 

 gest themselves to you. 



With reference to Prof. Moser's slow plates giving better effects than 

 rapid plates, that has not been our experience. We gave up the most 

 rapid plate. We experimented with the most rapid plate that we could 

 get, and we found some twenty of the plates were apparently light-struck ; 

 and finally we settled the question they were not light-struck ; they 

 were electric-struck by the brush discharge at the lower end of our 

 Crookes tube. 



One thing is very apparent to all of us that have b3en doing much work 

 in this line — that the induction coil needs improvement. For as at present 

 constructed they are not made to run continuously for twelve hours. 

 They are all right to run for a few moments for showing off Crookes 

 tubes ; but platinum terminals soon wear out or become hot ; and we 

 have to put on new ones. In that connection I have a very good idea, due 

 to a mechanic who does a great deal of work for me, which I will show by 

 a sketch. The platinum point is about a quarter of an inch long ordinarily 

 and is attached to the end of a tube having a thread on it and gradually 

 wears away. Instead of fastening that piece of platinum directly under 

 the tube we take a piece of platinum wire four or five inches long and 

 place it on the end of a second metal rod which screws into the first. In 

 that way, instead of having simply a quarter of an inch of platinum to 

 wear off, we have some four or five inches at our disposal ; and in the 

 next place the heat is dissipated long before it gets to the soldered joint. 



I think in connection with these photographs, there are shadow photo- 

 graphs ; but it is remarkable what an amount of detail we can see on 

 some of it. I have a photograph of a razor taken inside of the case which 

 is interesting to see. When we looked at it, it was very briglit in the 

 middle of the razor — more light coming through there than at the edge. 



