752 REPORT—1890, 
TUESDAY, SEPTEMBER 9. 
The following Papers and Reports were read :— 
1. Optique minéralogique.—Achromatisme des Franges. 
By Professor E. Mascarr. 
2. Instantaneous Photographs of Water Jets. 
By Lord Rayuuicu, Sec. B.S. 
These photographs were taken by the light of the electric spark. A battery of 
Leyden jars was charged by a Wimshurst machine, and discharged itself between 
brass balls, held almost half an inch apart, in the optical lantern. By means of a 
large condenser a good proportion of the light was concentrated upon the lens of 
the camera. The jet of water, regularised by a tuning-fork, fell in front of the con- 
denser, and was focussed upon the photographic plate. 
In the absence of anything to diffuse the light, the pictures are simple shadows, 
such as have been obtained without any optical appliances by Mr. Bell and Mr. 
Boys. The only detail is due to the lens-like action of the jets and the drops into 
which it is resolved. This arrangement is quite sufficient to illustrate the behaviour 
of electrified jets. But the interposition of a plate of ground glass close to the con- 
denser effects a great improvement in the pictures by bringing out half tones, and 
the results printed on aristotype paper are -now very good. The only difficulty is 
that due to the loss of light. In some of the experiments it was found advantageous 
to diminish the diffusion by slightly oiling the ground glass. 
The degree of instantaneity required depends upon circumstances. In some 
cases the outlines would have lost their sharpness had the exposure exceeded 
rvoop second, It is probable that the actual duration of the principal illumination 
was decidedly less than this. 
3. Report of the Commvittee on Electrical Standards, including the four 
following Papers.—See Reports, p. 95. 
4. On Variations in some Standard Resistance Coils. 
By R. T. Guazesroox, F.R.S.—See Reports, p. 98. 
5. On some Standard Arr Condensers. By R. T. Guazesroox, F.R.S., and 
Dr. A. Murruzap.—See Reports, p. 102. 
6. On the Specific Resistance of Copper. 
By T. C. Firzparricx.—See Reports, p. 120. 
7. A Comparison of a Platinum Thermometer with some Mercury Thermo- 
meters. By K. H. Grirriras.—See Reports, p. 180. 
8. On the Character of Steel used for Permanent Magnets. 
By W. H. Preucs, F.R.S. 
The quality of English magnet steel having apparently deteriorated, and being — 
much below that of France, led the author to make an exhaustive inquiry into the 
comparative merits of each kind. Samples were obtained from all the leading 
