184 T. F. SMITH ON THE USE OF TSOCHROMATIC PLATKS. 



light-filter in conjunction with it, the impression conveyed is 

 that the sharpness of the resultant image depended upon such 

 screen or light-filter. To take one single instance. At the 

 last meeting of this Club Mr. Lees Curties exhibited a bottle 

 to hold solutions for giving approximately monochromatic light 

 — such as, for instance, copper chromic solution — and in the 

 discussion which followed Mr. Houghton Gill said, that with 

 this solution and isochromatic plates he had obtained as good, 

 or almost as good, results with a cheap achromatic, as he had 

 been able to do with an apochromatic objective, used without 

 the absorption fluid. 



Now, I cannot help feeling that there is some confusion 

 here between cause and effect, and it is to the isochromatic plates, 

 and not to the solution, that is due the fact that the image in 

 the negative came out in the same plane as the one placed on 

 the screen, even when ordinary achromatic lenses were used. 

 I agree with all that Mr. Grill stated with regard to the results 

 obtainable with ordinary lenses, but the evidence of the effect 

 of the solution to me is not conclusive, unless it can be also 

 shown that the results were different when the isochroma,tic 

 plates were used only without any ray-filter. I do not wish for 

 a moment to deny that any monochromatic light will make a 

 great difference in focus when ordinary photographic plates 

 are used, but it is isochromatic plates we are here dealing with, 

 and I wish to prove that, when these are used, no light-filter 

 whatever is required to produce sharpness of focus ; and for 

 this purpose I beg to exhibit prints and negatives taken first on 

 isochromatic, and then on ordinary plates to show the difference 

 of result. The lenses used were all by one maker — Swift 

 and Son — and I believe that the Jena glass is used, but no 

 fluor spar ; but I do not wish to imply from this that other 

 makers' objectives will not give the same results. 



Prints Nos. 1 and la show the proboscis of Blow-fly, taken with 

 an inch objective at 300 diameters, first on an isochromatic 

 plate, and then on an Ilford ordinary, and you will see that 

 while the first is sharp in focus, the second is all fluff. I may 

 say in justice to this lens that there is but little divergence of 

 focus when used photographically up to 50 diameters on any sort 

 of plate, but that does not vitiate my argument that what- 

 ever difference may exist is corrected by the use of an isochro- 



