82 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLET 
very weak, and is far surpassed in strength by many substances whose specific 
gravity is about the same. Fused quartz, for instance, is much stronger, and several 
discs have been cut from this substance, and are awaiting manufacture. We are also 
trying with slices of bloodstone, &c., and expect to obtain a considerable reduction in 
weight. It must never be lost sight of that the weight of the mirror is an unmixed 
evil. The mirror in use at present is of glass silvered on the back, with a diameter of 
I'l centim. and a weight of '0485 gram—it is slightly concave, having a radius of 
curvature of one metre, which was measured approximately. When we discuss the 
lines along which further improvements can be made it will be shown that owing to 
a peculiar mode of scale illumination which we have discovered, these dimensions can 
probably be a good deal reduced, but as our coils are "wound at present, no advantage 
would be gained by effecting this reduction. The mirror is supported above the 
magnet system, but we are not sure that this is a good arrangement with such a 
large mirror—with a smaller one it would not matter. The definition of the mirror* 
is so good that the image of a millimetre scale in it is read at a distance of *2'67 metres, 
by an improvised telescope and micrometer eyepiece, whose scale is graduated to 
"2 millim. Each division of this scale can be divided by eye into about 5 parts; 
and by means of the diffraction fringes surrounding the scale images, the latter can 
be located to this degree of accuracy in the micrometer. 
With regard to the designing of the instrument we will take the conditions for 
maximum sensitiveness as laid down above, in order. We have experienced so much 
inconvenience ourselves when reading papers, in having to refer from lettering in the 
text to letters on the drawings, that we have prepared notes explanatory of the 
drawings to be placed beside them, and will endeavour here to give what explanations 
are required without making it necessary for the reader to refer to the drawings at 
every moment. 
(1.) The magnetic force for unit current should he a maximum —resistance of the 
coil, or trouble and expense of winding, not to be considered. We have endeavoured 
to make an improvement in this direction, by using four pairs of coils, one above the 
other, instead of two, as has hitherto been the case. Preliminary trials showed us 
that we could hope to make the suspended system about 20 centims. long, and con¬ 
sequently the maximum diameter of the coils was approximately fixed. It will be 
noticed that the coils are much smaller than those ordinarily employed. This may be 
taken as a realization of a well-known principle, that when it is a question of resis¬ 
tance or quantity of wire, it is better to have many small coils than few large ones. 
We began by making a drawing of an instrument with eight tiers of coils, but aban- 
doned it on account of the difficulty of making a suitable magnetic system of the 
necessary length. It is necessary to wind the wire regularly at the commencement of 
the winding, in order to get most turns in the most important part. We decided to 
wind the coils with cylindrical holes. The dimensions of the coils are shown in the 
* This mirror was made for us by our assistant, Mr. J. Cook. 
