870 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the Zeiss apochromatics are not visually superior to first class achroma- 
tics of some other makers ; but it is also admitted that they have usually 
given better results photographically. 
A few months since I made a critical examination of the colour 
corrections of my Zeiss 2-mm. apochromatic and I found it slightly over- 
corrected. The same was true of two other Zeiss apo’s. My Wales 
1/4 in. was also over-corrected. Reflecting on the facts herein narrated 
1 was led to the provisional induction that to get the best results with 
orthochromatic plates the lenses should be over-, rather than under- 
corrected. 
To confirm or refute this induction I next tested the 4/10-in. Wales 
and No. 4 Hartnack, both over-corrected, and found that their per- 
formance was much better witli orthochromatic than with plain plates. 
There was, however, something still lacking ; the negatives did not 
possess the absolute sharpness that was desired. The defect in definition 
especially noticeable at the margins of the picture I attributed to unequal 
magnification by rays of different refrangibility. To overcome this I 
excluded the blue, violet, and ultra-violet rays by means of a suitable 
ray-filter. The one that proved the most satisfactory was a solution of 
tropseolin (Grubler’s 000). This permits the passage of the red, orange, 
yellow, and a portion of the green. The advantages gained by its use 
were great, but still were not equal to theoretic demands. To carry out 
these practically to the fullest extent will require an absolute harmony 
in illumination, in lens, and in plate, each being adjusted so far as 
possible to the rays of the same refrangibility. In former times the 
photographic plates then in use were sensitive only to the G, H, and 
ultra II regions of the spectrum, and lenses were under-corrected in 
order to bring the more refrangible actinic rays into coincidence with 
the more powerful visual ones from the D region. 
Insomuch as the present orthochromatic plates are capable of giving 
results utterly unattainable on plain plates, I am satisfied by practical 
experience as well as theoretically, that the photomicrography of the 
future will advance considerably over that of the past if the D region 
be selected as the standard for illumination, for the sensitiveness of the 
plate, and for the correction of the objective. The first may be secured 
by suitable absorptive solutions or the employment of monochromatic 
yellow light obtained by prismatic or diffraction dispersion. Plates 
specially sensitive to I) light are readily accessible, and there is 
therefore no difficulty in fulfilling the second indication. The third 
requirement will necessitate a slight modification of the formulas usually 
employed by the leading opticians. Mr. Herbert R. Spencer, son and 
successor of the late Chas. A. Spencer, is now making for me a 1/6 
homogeneous immersion specially corrected to the end in view.” 
Nachet Photomicrographic Apparatus. — The instrument shown in 
fig. 98 consists of the large model inverted, with camera attached to 
the side-tube. The distance between objective and eye-piece amounts 
to 1*20 m. The mirror is silvered, so that the loss of light is insig- 
nificant, and the highest objectives can be used. 
By reason of its absolute stability and the facility which it offers for 
very oblique illumination, this instrument is particularly adapted for 
photographing diatoms. The focal adjustment and arrangement of 
