368 Scientific Proceedings, Royal Dublin Society. 
points I will not attempt any description, but merely mention a 
few points in which these instruments will be unique. 
The Transit Circle—A transit circle of 5 inches in diameter is 
now in preparation. The peculiarities of this instrument will 
consist iIn— 
Ist. An important modification in its mechanical construction, 
by which it is confidently hoped greater stability will be 
obtained, and exemption from some annoying and troublesome 
effects of flexure, which are at present in almost all large imstru- 
ments of this class; and 
2nd. The adoption of glass as a material for the circles, instead 
of brass and other metal. 
This last modification will be a very serious one, and I look 
anxiously for the result, but have every confidence of success. 
My reasons for making this radical change in the most important 
part of the most conservative of all astronomical instruments are 
twofold. 
Ist. Because the perfection and permanence of such circles 
depends much on the state of internal strains and molecular con- 
dition of the material of which it is composed, and these strains 
can all be ascertained and corrected if need be in the glass, but in 
the metal there is no possibility of detecting them. 
2nd. Because the reading of divisions on glass is capable 
of much greater accuracy than in metal. Metal circles must be 
viewed by reflected light, and all observers know that even the 
flickering of the lamp causes an apparent change of position 
of the lines, but in transparent circles, illuminated properly from 
behind, this does not take place. 
In support of this view, I quote a passage from a paper by Mr. 
Rutherford, who, I suppose, may now be considered the greatest 
living authority on instruments of precision. This paper was 
sent to me some time after I had made my proposals for glass 
circles to the Cork instrument, and I was much pleased at such a 
remarkable corroboration of what I almost feared was too bold 
an innovation. 
Mr. Rutherford says :— 
‘¢T am convinced from the ease with which one second is read on my 
instrument, with microscopes only 4$ inches long, including objectives 
and eye-pieces, that upon a circle of 15 inches, provided with powerful 
