TRANSACTIONS OF THE SECTIONS. 37 



concerned; but I have not m yet oxpeiimented on the spherical aberration, nor am 

 I so sanguine of its success. 



There sceras another direction in which a possible advantage might be gained 

 by use of these correcting combinations, viz. in the case of minute stars wliose 

 light is made up for a great part of rays from either end of the spectrum, more 

 particularly the blue end. It seems highly probable that better definition of these 

 stars could be obtained if a slight temporary adj ustment could be made in the 

 chromatic correction suitable for that particular part of the spectrum from which 

 the predominant light of the stai's proceeds. 



Of course it is to be understood that the corrections here spoken of and proposed 

 to be dealt with by their correcting lenses are ojily the very final ones — in fact, when 

 the objective arrives at that degree of perfection in which it is almost impossible 

 to say whether any improvement can be effected or not. 



On Ilecent Improvements in Equatorial Telescopes. 

 By HowAED Gkubb, Master of Engineering, Trinity College, Buhlin, 



The author referred to former papers read by him at the Brighton and Belfast 

 Meetings of the Association on the same subject, and proceeded to describe — 



1st. A method of conveniently reading the R.A. circle from the eye-end of the 

 telescope, 



2nd. A new simple but effective arrangement for .slow motion in R.A. 



Svd. A new and very much improved, form of clamping arrangement for both 

 polar and declination axes. 



4th. And a new method of controlling the uniform motion driving-clock of the 

 telescopes from an ordinary sidereal clock by an electric current transmitted once a 

 second from the sidereal clock; by which arrangement the driving-clock can be 

 kept going continuously without the possibility of accumulation of errors beyond a 

 small fraction of a second. 



On a Method of Phofogra piling the Defects in Optical Glass arising from want 

 of Jlomngenetfy. By Hovvakd Grubb, Master of Engineering, Trinity 

 College, Duhlin. 



The best practical method used for detecting in disks of optical glass defects 

 arising from want of homogeneity is probably well known to many amateurs as well 

 as to professional opticians. 



The disk of glass to be examined should be either itself polished to a convex form, 

 or, if that be not convenient, it should be placed in juxtaposition with a piece of 

 glass which is known to be perfect and of such form as will i-ender the combination 

 of the two of convex power. A small light (say gas- or candle-flame, or any suf- 

 ficiently brilliant light with a small diaphragm in front, see fig. 1) is placed at some 



Fig.l. 



little distance, and the eye is placed in the conjugate focus formed by the lens of 

 this light. The disk of glass should then appear brilliantlv illuminated ; but if 

 the pupil of the eye is drawn slightly to one side, so that the pencil of light falls 

 upon only one half of the pupil, immediately and most distinctly almost any want 

 ot homogeueilv is easilv seen. 



