788 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of necessity be nnder-corrected, as already stated. In accomplishing 
this, however, there is a certain loss of visual excellence which, however, 
is of little moment in ordinary photography. That this under-correc- 
tion of lenses for photomicrography results in an impairment of their 
visual excellence is well known to opticians, bnt has thus far received 
but little notice from the actual users of the lenses in question. A few 
recent writers, however, have directed attention to the matter. 
Londe * says : ‘ The first and most important question is the choice 
of objectives. These may be excellent for observation and more than 
mediocre for photography.’ 
Mercer, f in speaking of some of the objectives used by him in photo- 
micrography, says. ‘ The Wales objectives are corrected spherically for 
the violet ray. The violet image is, therefore, somewhat superior to 
the visual, with which, however, it is coincident.’ 
Czapski, in a letter published in Van Heurck’s treatise on ‘The 
Microscope,’ London, 1893, says, ‘ In every case the objectives specially 
constructed by opticians for photography can never be advantageously 
employed for observations, and inversely.’ 
From the foregoing it will be clear that lenses which were best for 
visual purposes were not the best for photographic use, and it was 
necessary, therefore, when the most perfect results were sought in both 
departments, to have a double set of objectives, and many investigators 
did provide themselves with such an outfit.* 
The inconvenience and lack of economy involved in this arrangement 
is manifest, but how to obviate it does not appear to have occurred to 
opticians or others interested in the subject. 
Having been practically familiar with photography and photomicro- 
graphy for upward of twenty years, I have had the opportunity in that 
period to become reasonably familiar with the inherent defects of their 
technique, but it was not until the latter part of 1891 that I perceived 
that a way out of the difficulty might be readily found. 
During the past five or six years the manufacturers of gelatino- 
bromide plates have placed on the market plates which are extremely 
sensitive to yellow light, and but feebly so to the blue, violet, ultra- 
violet rays of the spectrum, as will be perceived on examination of the 
band marked 26 (see fig. 112), and comparing it with band marked 1, of 
the same figure. (The other bands do not concern the purposes of this 
paper). 
If now the objective be corrected for yellow instead of for blue or 
violet light, the negative being made on one of these yellow sensitive 
or so-called ‘ orthochromatic ’ plates, there should be an exact corre- 
spondence of the visual and chemical foci, and the resulting picture 
should be superior to one that could be obtained by the ordinary pro- 
cedures — that is, an under-corrected lens and blue -sensitive plate ; and 
at the same time the objective would not have its visual excellence 
impaired, supposing, of course, that the optician performs his part of the 
work with care and skill. 
* ‘La Photographie Medicale,’ Paris, 1893. 
t Joum. Roy. Micr. Soc., June 1892. 
X It must be admitted that some of the apochromatics of short focus (2 mm.) 
obviate, in a measure, this difficulty, but those of longer focus have not, at least in 
my hands, proved satisfactory. 
