Marcu 7, 1907] 
NAT EOL LE 
451 
The abnormality of the distribution of the estimates now 
becomes manifest, and is of this kind. The competitors 
may be imagined to have erred normally in the first 
instance, and then to have magnified all errors that were 
negative and to have minified all those that were positive. 
The lower half of the ‘‘ observed’’ curve agrees for a 
large part of its range with a normal curve having the 
p.e.=45, and the upper half with one having its p.e.=29. 
I have not sufficient knowledge of the mental methods 
followed by those who judge weights to offer a useful 
opinion as to the cause of this curious anomaly. It is 
partly a psychological question, in answering which the 
various psychophysical investigations of Fechner and others 
would have to be taken into account. Also the anomaly 
may be partly due to the use of a small variety of different 
methods, or formulz, so that the estimates are not homo- 
geneous in that respect. 
It appears then, in this. particular instance, that the 
vox populi is correct to within 1 per cent. of the real 
value, and that the individual estimates are abnormally 
distributed in such a way that it is an equal chance 
whether one of them, selected at random, falls within or 
without the limits of —3-7 per cent. and +2-4 per cent. 
of their middlemost value. 
This result is, I think, more creditable to the trust- 
worthiness of a democratic judgment than might have 
been expected. 
The authorities of the more important cattle shows 
might do service to statistics if they made a practice of 
preserving the sets of cards of this description, that they 
may obtain on future occasions, and loaned them under 
proper restrictions, as these have been, for statistical dis- 
cussion. The fact of the cards being numbered makes it 
possible to ascertain whether any given set is complete. 
Francis GALTon. 
THE WORK OF THE OPTICAL SOCIETY.} 
“[ HouGH it is perhaps seldom that the Transactions of 
the Optical Society contain much record of original 
research, yet they often furnish matter of considerable 
value to the practical optician, and will usually be found 
to contain more than one paper of importance to the 
student of optics. The first paper in the present volume 
will have interest for many readers. It is a short and 
clear statement, by one well qualified to judge, Mr. W. A. 
Dixey, of the case for the use of periscopic lenses in 
spectacles. A periscopic lens, as defined by Mr. Dixey, 
is one through which its wearer can look obliquely as 
well as directly without his vision being impaired by 
radial astigmatism. The result is attained either by 
deepening the curves of the lens so as to produce an 
approximation to a sphere the centre of which coincides 
with the centre of rotation of the eye, or by the use of 
toric lenses. A careful reading of Mr. Dixey’s paper would 
probably lead many wearers of spectacles to pay another 
visit to the optician. 
The paper by Mr. Theodore Brown, on direct stereo- 
scopic projection, is of special interest. It describes an 
ingenious device for obtaining stereoscopic effects in pictures 
projected on a screen, of which probably more will be 
heard. The argument is that in binocular vision the 
stereoscopic effect is due to the fact that the two images 
of the same object in the two eyes are not formed in 
similar positions on the retina, and that it should be possible 
to produce stereoscopic effects even when one eye only is 
used if by some means two simultaneous pictures can be 
formed on the retina in appropriate positions. Owing to 
the permanence of retinal impressions, this can be effected 
by throwing on the screen with rapid alternation the two 
stereoscopic pictures in somewhat displaced positions. The 
displacement is produced by giving a motion to the pro- 
jection camera, and the stereoscopic can be combined with 
the ‘‘ bioscopic’’ effect by the use of a kinematograph 
mechanism. It is clear from the paper and the discussion 
that further perfection of detail is necessary to produce 
completely satisfactory results, but there seems no reason 
why the difficulties should not be overcome. There is, 
1 ‘Transactions of the Optical Society, London, Session 1904-5. Pp. 93. 
Price ros. 
NO. 1949, VOL. 75] 
however, some reason for suggesting that perhaps a one- 
eyed spectator would be the most appreciative. 
In ‘‘ A Method of Testing Prisms,’’ Mr. S. D. Chalmers, 
the head of the optical department at the Northampton 
Institute, suggests some useful applications of the method 
of auto-collimation for the determination of the angles of 
prisms to the highest possible accuracy. We would direct 
attention especially to the procedure suggested for the 
measurement of one of the angles of a 60° prism apc. 
Rays entering perpendicular to the face Be are totally 
reflected at 30° from ca or ap, and reflected normally at 
AB or CA, emerging again approximately perpendicular to 
bc. From the separation of the incident and emergent 
rays the error in the angle a can be determined. Only 
one reflection takes place normally at a glass-air surface, 
and there is, therefore, no difficulty in seeing the image. 
Simultaneous observation of the direct reflection from Bc 
aids in setting the prism. A similar method can readily 
be applied in other instances, and the figures in the paper 
suggest at once the procedure in the cases which occur 
most frequently. “Che lack of parallelism in plane parallel 
glass can also be tested in this manner. Rigidity is, of 
course, essential in the apparatus to be employed for the 
observations, and Mr. Chalmers has obviously given some 
attention to details in the construction of a suitable auto- 
collimator ; drawings or diagrams of this would have added 
to the value and interest of the paper. 
Mr. L. W. Phillips, student member, in a paper on 
the measurement of absorption in tinted glasses, describes 
some observations on the absorption of light by coloured 
glasses, such as are used for spectacles, a “‘ flicker ’’ photo- 
meter being employed for the measurements. The method 
does not admit of great accuracy, but is no doubt useful 
within certain limits for rough work of the kind sug- 
gested. It raises, however, some vexed questions as to 
the photometry of coloured lights, and some points of 
interest in relation to the ‘‘ flicker’’ instrument were 
brought up in the discussion by Dr. Garnett and others. 
A presidential address by Prof. Silvanus P. Thompson, 
on the early literature of optics, demands no more than 
passing mention here, great though its interest must have 
been to those who had the pleasure of listening to it. 
The volume closes with a contribution by Mr. A. T. Bull, 
entitled ‘‘ Some Notes on the Nature of Vision.’’ The 
paper, being essentially of the mature of notes, passes 
rapidly over many points of interest in connection with 
the mechanism of vision and the molecular and other 
phenomena accompanying it. Various topics are thus 
touched on, from the accuracy of the photometric results 
obtained with rotating sectors to the difficulties yet re- 
quiring explanation on the Darwinian theory in regard to 
the process of evolution of vision. The notes are sug- 
gestive, but would make more interesting reading if less 
loosely put together. 
We would venture to suggest, in conclusion, that the 
editing of the volume leaves something to be desired. It 
may be possible to interpret such phrases as ‘‘ beep lenses 
on the Willaston principle,’? and to escape being deceived 
by the “‘ dissimulation ’’ of a photochemical body, but the 
fact that Lewis Carroll was a mathematician does not 
sufficiently justify such imitation of his playful extrava- 
gances in a scientific journal; and the volume is marred by 
many such misprints. 
THE COMPRESSIBILITY OF CRYSTALLINE 
ROCKS: 
THE latter-day revival of interest in geological physics 
has led to a keen demand for experimental data, the 
absence of which has hitherto rendered futile most specula- 
tion in this domain. Our almost complete ignorance of 
the simplest physical constants of rocks and the rock- 
forming minerals is easy to account for. The kind of 
investigation required is both difficult and _ laborious, 
calling for skill and practice as well as the appliances of 
a well-equipped physical laboratory ; and the geologist may 
lack either the capacity or the opportunity for such re- 
1 ‘* An Investigation into the Elastic Constants of Rocks, more F specially 
with Reference to Cutic Compressibility.” By Prof. Frank D. Adams and 
Prof. Ernest G. Coker. Pp. 69. (Washington, D.C. : Carnegie Institution, 
1926.) 
