340 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Keeley, E. J.— Neglected Feature in the Construction of Achromatic Condensers. 
[The author suggests the employment of a correction collar with an achromatic 
condenser. A condenser with adjusting collar has been designed by Mr. 
E. M. Nelson, and described in this Journal, 1895, p. 230, fig. 32.] 
Micros. Bull. (Philadelphia), 1899, pp. 5-6- 
„ „ Some simple Methods for producing Vertical Illumination. 
Micros. Bull. (Philadelphia), 1899, pp. 7-8. 
(6) Miscellaneous. 
Early Achromatic Telescope. — The President has sent us fig. 90, 
and the accompanying description of a very early achromatic telescope 
in his possession made by John Dollond. 
This telescope is interesting from the circumstance that the achro- 
matism of the Microscope was derived from that of the telescope. The 
telescope was first achromatised by Mr. Chester Moor Hall in 1733, 
but he appears to have kept the invention to himself, as he did not 
publish anything about it. 
Fig. 90. 
Achromatism was independently discovered by John Dollond, a 
Spitalfields silk-weaver, who studied mathematics and optics, and who 
gave up the weaving business in 1752 to join his son Peter Dollond, an 
optical instrument maker in Vine Court. 
John Dollond, in 1757, investigated the laws of dispersion, and the 
next year the Copley Medal of the Royal Society was awarded to him 
for his invention of achromatism. As John Dollond died in 1761, the 
date of this telescope may be placed between the years 1758 and 1761. 
It is 4 in. in aperture, and 10 ft. in focus. The body is composed of 
three square mahogany tubes, which slide one within the other ; there 
are spring bolts fixed to two of them which shoot when the tubes are 
either fully extended or quite closed. The eye-piece is a Huyghenian, 
with a push tube focussing ; the power is 100. 
The stand is somewhat peculiar. In the centre of the tripod^there is 
