TRANSACTIONS OF SECTION A. T27 
what has been already won as in securing new conquests. I hope that the apathy 
of so many years may lead to a splendid awakening in this country, and that our 
past neglect of this most beautiful theory may be atoned for in the future by 
special devotion and appreciation. 
The following Reports and Papers were read :— 
1. Report of the Committee on Hlectro-optics.—See Reports, p. 144. 
2: Notes on High Vacua.! By J. SwInBorne. 
I. 
A form of Geissler pump is described, in which the head of the pump delivers 
into an exhausted chamber, whose valve is opened automatically so that there is no 
_ back pressure due to small condensations of air or vapours. 
II. 
Experiments with two M‘Leod gauges were made to find the tension of mercury 
vapour. Roughly, the pressure is about fifty-one millionths of an atmosphere, so 
that a mercury pump cannot produce vacua of one millionth, or one three-hundredth 
of a millionth, as commonly supposed. 
3. On the Use of the Lantern in Class-room Work. 
By Professor Arcu. Barr, D.Sc., and Professor W. Stroup, D.Sc. 
After referring to the advantages of lantern illustrations as compared with 
diagrams for class lectures, the authors described arrangements whereby the lantern 
may be used in partially lighted rooms during the daytime, and in the evening 
without diminishing the ordinary illumination. In these cases the lantern-screen 
is placed so that no direct light falls upon it. The lantern is placed on the lecture- 
table and operated by the lecturer. 
The authors described a simple and convenient form of lantern to be used for 
horizontal or vertical projection. When using vertical projection for exhibiting 
lantern slides, the slides are seen by the lecturer in their proper aspect (z.e., not 
inverted nor turned right for left), so that any details may be readily indicated by 
‘a small pointer on the slide itself. By using a finely ground and oiled glass-plate 
in place of the lantern slide, the lecturer may write or sketch upon it instead of on 
the blackboard. 
\ An apparatus was exhibited for the preparation of lantern slides in large num- 
_ bers from illustrations in books, periodicals, &c. This consists of a book-holder 
having provision for enabling the operator at once to set the picture centrally oppo- 
_ site the camera. A carriage, movable along a railway, supports the camera by a 
mechanism which permits it to be adjusted to any desired height, but constrains it 
so that it is always horizontal and properly directed. A scale upon the railway 
and upon the base of the camera graduated in accordance with the size of the illus- 
trations to be photographed enables the operator at once to set the camera at the 
roper distance from the picture, and to focus it; thus, if the picture be 15 inches 
ong the carriage is moved to the graduation 15 on the railway and the back of the 
camera to a graduation 15 on the base. The height of the camera also is adjusted 
by a scale in accordance with the reading of a scale placed on the book-holder at 
the side of the picture. By these means the camera is completely set and focussed 
without the use of a ground-glass screen, The picture is illuminated by gas or 
other artificial light. 
1 Electrician, Sept. 5, 1890. 
