PROCEEDINGS OF THE SOCIETY. 
277 
Piffard’s letter on the above subject, read here on 19th October last, 
contained valuable information for those who using ordinary achromatic 
objectives photographically have to struggle with the difficulties of the 
want of identity between the visual and chemical foci of such lenses, 
and had he finished by describing the happy result when isochromatic 
plates were substituted for the ordinary ones, there would have been 
nothing left for me to say ; but when he goes on to mix up the question 
of focus with various light-filters, he seems to me to render difficult a 
very simple operation, and his remarks subsequently would tend to 
repel rather than to attract those who wish to record their results by 
photographing them. 
Dr. Piffard mentions his disappointment when using certain lenses on 
the ordinary wet plates some fourteen years ago, and afterwards on the 
commercial dry ones, although the same lenses gave a good image visually, 
and then goes on to say how some ten months ago, when trying a new 
1/4 in. on a commercial isochromatic plate, the result was a gratifying 
surprise. Up to this point there is no reference to any screen, and the 
perfection of the photographic image seems from the internal evidence 
of his letter to be due to the use of the isochromatic plate only, but when 
reverting to the old glasses again, and using them on the same sort of 
colour correct plates, he finds their performance much better than of old, 
but still lacking the absolute sharpness desired ; and then to overcome 
this he excludes certain rays by means of a suitable ray-filter, finds the 
advantage gained by its use very great, but still not equal to theoretic 
demands ; and at this point we are left to struggle with our difficulties 
as best we may until the author has discovered certain new combinations 
of light, and certain new corrections in objectives which shall produce 
the desired result ; which will arrive — to use his own words — ■* When 
there is an absolute harmony in illumination in lens and in plate, each 
being adjusted as far as possible to the rays of the same refrangi- 
bility.’ 
Now, working as I have been on the same lines as Dr. Piffard for the 
last twelve months as far as experimenting on isochromatic plates is 
concerned, all these refinements of illumination and correction he deems 
necessary to secure a perfect photographic image certainly fill me with 
astonishment, having myself found no difficulty in producing a perfectly 
sharp picture of the object with such plates without using any screen or 
light-filter whatever, and my opinion is that any such appliances have- 
no effect on focus. 
I do not deny that monochromatic light may affect focus to a great 
extent, when ordinary achromatic lenses are used photographically on a 
commercial plate not colour correct — indeed we have plenty of evidence- 
to that effect ; neither do I deny the advantage of certain screens in 
rendering colours in the object more or less actinic, All this I admit, 
but when referring to all the remarks I can find on the subject, I see 
the authors speak of isochromatic plates only as being used in connection 
with the light-filters, and argue from it that it is to the latter is due 
the sharpness of the resulting picture. 
I expressed the opinion that there was a confusion here between cause 
and effect, in a paper read by me in November last before the Quekett 
Club, but since then, having read Dr. Piffard’s letter, I have made special 
