500 Transactions. 



have been mounted, as a few minutes' scrutiny of a large star will show at 

 once whether the figure is good or bad without removing the mirror fi-om 

 its tube. A high power must be used for large mirrors. Very large mirrors 

 seldom have absolutely first-class figures for dividing double stars, but are 

 sufficiently good for photography, as the halation hides the minor faults 

 which make the difference between a perfect mirror and a good one. Until 

 mirrors are figured with the same care as achromatics they will never give 

 satisfaction ; and I may add that a perfect flat is as essential as a perfect 

 mirror. If the flat is concave it will give, slightly out of focus, an oval 

 disc of light instead of a circle, and quite spoil the definition. 



In conclusion, I should like to acknowledge my debt to Mchol's Cyclo- 

 paedia, published in 1837, and to Cooke's book on " Telescopic Objectives." 



Art. LII. — Southern Variable Stars. 



By C. J. Westland, F.R.A.S. 



[Bead before the Astronomical Section of the Wellington Philosophical Society, 



4th August, 1915.] 



Ix choosing " Southern Variable Stars " for my title I wish to explain that 

 I am referring to stars which are well situated for observation in New Zea- 

 land, but without excluding some which may be visible to a certain extent 

 in Europe also. The southern stars are not nearly so well known as the 

 northern ones, and this is true of the variable as well as the other stars. 

 Still, there is much to be learnt about variable stars in both hemispheres, 

 and with reference to this I may quote a statement made by Professor 

 Pickering in addressing the British Astronomical Association in June, 1913. 

 Out of 4,525 variable stars now known, 3,371 have been discovered at 

 Harvard College Observatory ; and out of all this number probably less 

 than a hundred have had their periods and ranges of magnitude determined. 



The explanation of this is that a discovery is made by comparing two 

 photographs taken on different dates ; but determination of a star period 

 requires prolonged observations for which no professional observer has time 

 to spare. Only amateurs can find the opportunities to collect the infor- 

 mation, and, fortunately for them, the work does not require very elaborate 

 instruments. The Variable Star Section of the British Astronomical Asso- 

 ciation has done much valuable work in this direction, and several of its 

 most diligent members have only 3 in. telescopes. When a star reaches 

 naked-eye magnitude a pair of field-glasses is more useful than a telescope, 

 because it may be necessary to compare the variable with a star of known 

 magnitude several degrees away. The field-glasses may be turned rapidly 

 from one star to the other in a way that is impossible with the telescope. 



A good example of the long-period variables is the star R. Hydrae. It 

 may be called a southern variable, for, although it is on the working list of 

 the B.A.A. Variable Star Section, the difficulties found in observing it made 

 it impossible to determine either maximum or minimum until a year ago, 

 when one member in South Africa and myself undertook observations of 

 it. Our results showed that at maximum it rose to 44 and at minimum it 

 fell to 9-3, so that at one period of its career it gives a hundred times as 

 much light as at the opposite extreme. 



My own attention was called to this star in an unexpected way. In 

 1911 a comet appeared which passed through Hydra in November, and was 

 among the comparison stars of R. Hydrae for several days. I exposed a 



