^0 Notes on a UemarJcahle Meteor seen at Ballan. 



Art. IV. — Notes on a Remarkable Meteor seen May 20th at 



Ballan. 



By Louis Le Gould, C.E. 



[Communicated 14th June, 1877.] 



Art. v.— Notes on the Design of Telescope Tubes. 

 By W. C. Kernot, M.A., C.E, 



[Read 12th June, 1877.] 



The problem which I desire to bring before the Society to- 

 night is that of the design of tubes for telescopes, and my 

 remarks will have especial reference to telescopes of large 

 size, such as for example the great Melbourne E-eflector. 

 These gigantic instruments are usually reflectors, and 

 generally consist of a large and a small speculum, with the 

 necessary subsidiary apparatus ; and the function of the 

 tube is to support these optical appliances in their correct 

 relative positions. Should the tube be of a fl.exible and 

 yielding nature, it will, by virtue of its own weight and 

 the weight of the specula, bend down or deflect when it 

 is in any position other than vertical; and this deflection 

 will vary in amount and direction in the various positions 

 the instrument is made to assume when directed to different 

 points in the heavens. Hence if the optical arrangements 

 are in correct adjustment in one given position of the 

 instrument, they will cease to be so when it is moved to any 

 other position. 



As all known materials are more or less elastic, it is 

 manifestly impossible to construct a telescope tube which 

 shall be altogether free from this objectionable deformation. 

 Nevertheless it is both possible and desirable to choose such 

 a material, and to arrange it in such forms, as to reduce the 

 inevitable deformation to a minimum ; in other words, it is 

 requisite to determine in what shape the material should be 

 arranged in order to attain a maximum of stiffness, and to 

 the question as thus limited I shall confine my further 

 remarks. 



