92 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
correctional lens adjustment for the aberrations arising from the thick- 
ness of the slip. I have a distinct affection for the Gillctt, for it was 
with that condenser I taught myself what a critical image was. In 
1874, however, I purchased a P. and L. new formula water-imm. 1/8 
of N.A. 1 • 21. These and similar lenses by Tolies far surpassed any- 
thing at that day. There was a greater difference between these lenses 
and their cotemporaries than there was between the homogeneous 
immersion and these same lenses four years later. I can only liken the 
improvement which those lenses ushered in to that which has lately been 
achieved by Abbe’s apochromatics. It was the possession of this lens 
(P. and L. new formula 1/8) that first made the inadequacy of the 
Gillett apparent to me. This led me to get P. and L. dry achromatic 
condenser, which I still have. This condenser was designed by Powell 
in 1857 ; it is a 1/5 in power, and *99 N.A. in aperture, and is the best 
ever introduced. 
I must now say a word or two on low power condensers. Low 
power objectives had, somehow or other, been left out in the cold, no 
condenser having been provided for them. A sop, in the shape of a 
paraboloid or spot lens, was every now and then thrown to them, but, 
as far as I know, the first low power condenser we hear of is Webster’s, 
in 1860. 
The next was Abbe’s chromatic,* 1873; Swift’s achromatic, 1874; 
Abbe’s achromatic, 1888, and Powell’s new one, last year. 
On the Continent the Microscope may be said to have remained 
condenserless until the rise of bacteriology compelled the general 
adoption of the Abbe in 1880. I will now give a parallel table showing 
the data and form of various condensers that have been introduced since 
the days of achromatism : — 
England. 
1826. Single lens, Tulley. 
1840. Objective. 
1850. Gillett, three pairs, N.A. '65. 
1857. Powell, two pairs and single, an- 
terior middle concave, N.A. 
•99. 
1865. Webster, single front, achromatic 
back. 
1874. Swift, two pairs and single front, 
N.A. -9. 
1878. P. and L. achromatic, improved 
anterior middle plane, * 99 
N.A. 
1881. P. and L. oil chromatic, same as 
Abbe only higher power, N.A. 
1*3. Ditto, truncated, N.A. 
1 -4. 
1887. P. and L. oil achromatic, N.A. 
1’4, three pairs and single 
front. 
1889. P. and L. low power achromatic, 
N.A. 1*0, one pair and two 
singles. 
* Abbe’s chromatic stopped down makes a far better low power condenser than 
it does a high power, as the stop reduces the abnormal amount of spherical aberra- 
tion. 
Continent. 
1827. Single lens, Amici. 
1833. Chromatic, Chevalier 
I 1839. Objective, Dujardin 
i 1873. Abbe, chromatic, N.A. 12, hemi- 
spherical front, crossed back. 
(? date) Another form, N.A. 14, single 
front, Herschelian doublet back. 
1888. Abbe achromatic, two pairs and a 
single front, N.A. 1 ‘0. 
