92 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
even the transverse strife of Amphipleura, are resolved with axial light. 
Pleurosigma angulatum shows new details which have still to bo studied. 
On examining it without the eye-piece eleven spectra are seen, i. e. five 
new intermediate spectra. Some bacteria have also shown new details. 
The illuminating power of the objective is very great. Strong 
photographs of the beads of Ampliipleura have been obtained in six 
minutes with a magnification of 1500 diameters with monochromatic 
solar light, uhilst with the ordinary apochromatics at least ten minutes 
was necessary with only 1000 diameters and ultra-oblique illumination. 
Only three objectives have yet been made, two for the Continental 
tube and one for the English. One of the former is in the hands of 
Dr. Koch, the Berlin bacteriologist, and will, it is hoped, give some 
interesting results. 
Dr. Van Heurck considers that “ the new objective forms an honour- 
able practical crowning of the long theoretical labours of the illustrious 
Prof. Abbe, who for fifteen years has so happily led the Microscope 
into new jiaths, and who has with indefatigable patience realized 
practically all that theory indicated.” 
Semi-apochromatic Objectives. — Mr. E. M. Nelson read the following 
note at the December meeting : — 
As these new semi-apochromatics are “ Students’ ” lenses, let me 
briefly trace their development. The earliest form of student’s lens 
was a combination of three “French buttons” or doublets; almost the 
whole of the medical and students’ work, both here and on the Continent, 
was carried on by means of these lenses. The one I am exhibiting 
to-night is an example of the favourite form in this country, the sale of 
which, as I am credibly informed, must be counted by thousands. This 
lens gave way to the Hartnack, which consisted of two doubles and a 
single. The Hartnack was an immense advance over the French button, 
but looking at them from a present day point of view, we should say that 
while some picked specimens were good, the bulk of them were very 
mediocre. Somewhat later came Seibert of Wetzlar. His lenses, in 
form not differing greatly from those of Hartnack, were decidedly 
superior to them in finish ; at the same time, his angles were low for 
the most part. Of his lenses two even now justly have world- wide 
celebrity. I allude to his No. III. and his water-immersion 1/16. 
Before leaving Seibert let me point out that a Seibert No. III., 
unscrewed from its brass mount, constitutes the best high-power pocket- 
lens ever made. One mounted like a Coddington would be a useful 
appendage to a microscopist’s outfit, as it has fully 1/8 in. working 
distance, which the Coddington has not. 
One other point. You are all aware that on the Continent almost 
nothing has been done with low-power lenses. Seibert alone of all the 
Continental makers produces a No. 0, which is a first-class 1^ in. 
With this lens Mr. Rousselet and myself have seen the cilia on Volvox. 
An example of this lens is on the table. The Hartnack was superseded 
by the Zeiss achromatic, a lens much of the same form, i. e. two doubles 
and a single, but altogether of superior workmanship. Zeiss also, by 
making each class of lens both wide and low angled, suited all tastes. 
To illustrate this period of lens I have brought a D D or 1/6 in. 
I have now come to the time when English opticians made students’ 
