- Dec. 28, 1882 | 
NATURE 
201 
advantages in supplementing the photographic records 
by direct eye-observations. I regret that the very few 
occasions on which it has been possible to observe the 
sun has put it out of my power to make further experi- 
ments in these and some other obvious directions. 
P.S.—[I have Capt. Abney’s permission to add the 
following letter this day received from him ;—“ A careful 
examination of your series of sun-photographs, taken with 
absorbing media, convinces me that your claim to having 
secured photographs of the corona with an uneclipsed 
sun is fully established. A comparison of your photo- 
graphs with those obtained during the eclipse which took 
place in May last, shows not only that the general 
features are the same, but also that details, such as rifts 
and streamers, have the same position and form. If in 
your case, the coronal appearances be due to instrumental 
causes, I take it that the eclipse photographs are equally 
untrustworthy, and that my lens and your reflector have 
the same optical defects. I think that evidence by 
means of photography of the existence of a corona at all 
is as clearly shown in the one case as in the other.”— 
December 15, 1882.] 
A WEDGE AND DIAPHRAGM PHOTOMETER 
A NEW photometer, shown in perspective in the figure, 
has lately been constructed by Mr. Sabine. The 
‘stand supports a straight horizontal tube, at one end of 
which is a paraffin lamp, and at the other an eyepiece. 
The middle portion of the tube is cut away, and has, 
slipped over it, a collar to which a frame is attached, 
carrying a wedge of neutral-tinted glass, adjustable by 
means of a rack and pinion. Inside the collar is fixed a 
transverse disc of ground opal glass, which the paraffin 
lamp illuminates to a definite degree. This disc consti- 
tutes the field of comparison, the illumination of which is 
adjustable by means of a series of diaphragms of known 
aperture at the end near to the paraffin lamp. At the 
side, between the wedge and the collar which carries it, 
is a narrow pane of ground opal glass, just behind which 
a small mirror is fixed at an angle of 45° to the axis of the 
tube. This mirror is supported from the centre of the 
transverse opal disc in such a way, that the support is 
hidden from the observer by the mirror itself, an arrange- 
ment which insures the apparent juxtaposition of the illu- 
minated surfaces which have to be compared. The light 
to be measured is placed on the right-hand side of the 
photometer ; andthe collar is turned so that the light falls 
normally upon the face of the wedge, passes through the 
wedge, through the pane of opal glass, and is incident 
upon the mirror, which reflects a portion of it to the eye 
of the observer. The wedge is then shifted, if neces- 
sary, to interpose a greater or less thickness of absorbing 
medium, until a balance is obtained, that is until the 
apparent illumination of the mirror is equal to that of 
the field of comparison, in the middle of which it is 
seen. Ifthe range of the wedge is insufficient to admit of 
this, the degree of illumination of the field is altered, by 
means of the diaphragms, and the wedge is then adjusted. 
The employment of glass wedges for photometric com- 
parisons is not new, having been already used by both 
Xavier de Maistre and Quetelet ; but no practical photo- 
meter based upon this method has hitherto been con- 
structed. The employment of diaphragms for extending 
the range of the wedge is found to work well and to 
enable the operator to adjust the illumination of the field 
with exactitude, the bright part of the paraffin flame 
being of course, kept opposite to, and so as to well cover 
the diaphragm aperture. A table is constructed giving 
for each position of the wedge and for each diaphragm, 
the value, in standard candles, of any light placed at a 
distance of one metre from the instrument; and if the 
light be placed at any other distance, the number in the 
table has simply to be multiplied by the square of the 
actual distance in metres. For ascertaining approxi- 
mately the amount of light which passes through any 
given coloured glass, for example, orange glass, the eye- 
piece is furnished with a rotary disc containing small 
panes of white and different coloured glasses, either of 
which can be interpoSed at pleasure. 
This photometer is being made by Messrs. Elliott 
Bros., in two forms, one for use as a portable photometer, 
as shown in the figure, and the other on a more solid 
stand, for laboratory purposes. 
ON THE OCCURRENCE OF GREAT TIDES 
SINCE THE COMMENCEMENT OF THE 
GEOLOGICAL EPOCH * 
T will I daresay be within the recollection of many of 
those who are now present that I was honoured by 
the invitation to deliver the opening lecture in this hall 
last year. In response to that invitation I addressed 
to you a discourse which I ventured to call “A Glimpse 
through the Corridors of Time.’’ Accounts of it have 
appeared in very many quarters, both at home and 
abroad. I am myself responsible for the account which 
appeared in the columns of NATURE, as well as for the 
pamphlet form in which the lecture has since been issued. 
The chief reason why I now recur to the subject remains 
to be stated. Among the various comments which have 
been made upon that address, some are by no means 
favourable to the views I ventured to put forward, and 
they have been the theme of considerable discussion. 
Up to the present I have not made any reply to the criti- 
cisms which have appeared ; I postponed doing so until 
a suitable opportunity should have arisen for a review of 
the whole subject. Your kindness in inviting me once 
again to address this great Institute has afforded such an 
opportunity, and with your permission I propose to preface 
the subject of my lecture this evening by a reply to those 
critics who have honoured me with their attention. 
Let me recall to you very briefly the subject of that 
lecture, so as to enunciate clearly the point as to which 
an issue has been raised. You will perhaps recollect that 
the lecture treated principally of the tidal relations 
between the earth and the moon, of the influence of the 
tides during ages past, and of the future which awaits the 
earth-moon system during ages to come. I pointed out 
that at the present moment the orbit of the moon must 
be gradually growing in size, that this gradual increase of 
the distance from the earth to the moon is essentially 
non-periodic, and thus is totally different to the ordinary 
lunar irregularities which are recognised in rigid-body 
astronomy. Asa consequence of this incessant growth 
in the moon’s distance we see that in past ages the moon 
must have been appreciably nearer to the earth than it is 
* Extract from a lecture delivered at the Midland Institute, Birmirgham, 
on November 20, 1882, by Prof. Robert S. Ball, LL.D., F.R.S. Conmimuni- 
cated by the Author. 
