414 TRANSACTIONS OF SECTION A. re 
With such a combination the ratio of effective aperture to focus reaches 
7°5 as against 12°5 for the astronomical triple above referred to, or, in other 
words, it has nearly three times the light grip. It is likewise free from 
secondary spectrum, but the remaining tertiary spectrum becomes rather 
more prominent when, the mean force having been expanded to 60 feet, 
a focal curve is drawn. For ordinary powers it is still difficult to detect. 
The radii of the spherical surfaces for such a triple are quite moderate, 
viz. : 
Ist surface . ‘ . . e . . » 3961 
2nd and 3rd s e « s e e . . Oo OUL 
4th and 5th a> tos o s . e . 1771 
6th . e s 7 t . « t « 27°174 
and the greatest curvature sum is :— 
1 
Ff _ —5:535 
—— = —-=—9 684. 
pw-1L 0572 pis 
Professor Hastings’s well-known requirements being that no one of the 
curvature sums should exceed 30. All inner surfaces are cemented. 
For spectroscopic purposes two of these triples (respectively for collima- 
tor and telescope) are placed at such a distance apart that their nodal 
planes are coincident, thus forming a symmetrical doublet. 
The lens shown was constructed for me by Messrs. Hilger, Ltd., of 
London. It is in use as a telescope lens, with power of 56 diameters, 
focussed on a test object, a suitable spectroscope not being at hand. Its 
aperture is 14 inch, its focus 114 inches. 
A doublet, designed for deer-stalking, of 15 inch aperture and 84 inches 
focus with a power of 40 diameters was also shown. 
7. On Magnetostriction. By H. G. Dorsny, Ph.D. 
What I wish to present is the result of recent rather exhaustive experi- 
ments on magnetostriction on a series of eight steel rods of known chemical 
contents and tested in different physical conditions. 
I have found that the maximum elongation is a function of the carbon 
content, the curve being similar to a curve in the iron-carbon phase 
diagram ; also that there is a rather definite relation between the maximum 
elongation and the maximum susceptibility of any specimen of iron, and 
the results of other workers seem to fit my curve pretty well, The maximum 
retraction in a given field bears an inverse relation to Young’s modulus. 
I also show that a correction should be applied to the changes in length, 
and that this correction is of the same nature as the correction to be applied 
in magnetic work to correct for the demagnetising effects of the ends of the 
rods. 
The upper and lower limits of my experiments include practically all 
the results fotind in the fifty or more articles published since Joule’s 
discovery of this phenomenon in 1842, and thus tend to unify the rather 
chaotic condition of conflicting experimental results. 
8. The Photographic Registration of Sound Waves. 
By Professor Drayron C. Minuer, D.Sc. 
For the quantitative study of the tone quality of musical instruments it 
seemed necessary to have photographic records of sound-waves which would 
be more accurate and more directly obtained than those heretofore used. 
——, 
