302 
MR. J. C. Me CONNEL ON AN EXPERIMENTAL INVESTIGATION 
polarised by the Nicol A and screens are placed so that no other light can reach the 
eye. The slit of the collimator has been removed so the light falls on the whole 
breadth of the object glass B of the collimator. Here it is transformed into a 
convergent beam which passes through the quartz plate C. Each parallel pencil 
emerging from the quartz is brought to a focus in the focal plane of the object- 
glass D. So the needle-point E has to be carefully adjusted to lie in this plane. In 
this particular the spectrometer has a great advantage over the polariscope. The 
focal length is much greater and so the adjustment can be made with greater accuracy, 
and the spherical aberration, which in the polariscope is very troublesome, is so small 
as to produce no sensible error. F is the eyeqfiece and G the second Nicol by which 
the light is analysed. 
I found that the ordinary eye-piece magnified too strongly, so I substituted a single 
lens and fitted it up with the Nicol G in a large cork, so that the combination could 
be readily exchanged for the ordinary eye-piece without disturbing the needle-point. 
Even with the low power the apparent size of the rings was vastly greater than with 
the polariscope. This was mainly due to the greater focal length D E, which was 
about nine inches. Though the definition of the rings suffered with the increased 
magnification, there was on the whole a decided gain in accuracy, and the observations 
became far less trying to the eyes. The spectrometer table too was furnished with 
a slow motion screw which was no small advantage. The spectrometer was fitted 
with a circle graduated on silver, and both the telescope and table were provided with 
verniers reading to half minutes. By replacing the slit and the ordinary eye-piece of 
the telescope the spectrometer could return to its ordinary use. The normal to the 
faces of the quartz could thus be set at right angles to the axis of rotation of 
telescope and table and its position determined relatively to the axis of collimation of 
the telescope for any given readings of the telescope and table verniers. For levelling 
purposes the quartz was mounted on a tripod stand resting on three adjustable screws 
whose rounded ends rested in a conical hole, in a V-shaped slot, and on a smooth 
surface, respectively on the table of the spectrometer. Each screw carried a loose nut 
which, when the screw had been properly adjusted, could be screwed down tight onto 
the tripod. This made the three screws a perfectly firm support and the tripod could 
be removed from the table and replaced accurately in its original position—a great 
convenience. 
The distance A B from the polarising Nicol to the object-glass of the collimator 
was over four feet, so the light which fell on the lens B had passed through the Nicol 
nearly as a parallel pencil, and therefore was polarised approximately all in one plane. 
The two Nicols could therefore be crossed with greater accuracy. In order to cross 
the Nicols the quartz was removed and the cork turned till the light was quenched. 
In my earlier observations I simply used the sodium light for this purpose, but I 
found that this led to errors of two or three minutes in the smaller rings, so the 
