500 Proceedings of Royal Society of Edinburgh. [sess. 
motion to the upper half-lens ; the other a two-and-fro horizontal 
motion to the lower half-lens. This latter motion, while desirable 
in itself to correct a possible maker’s error in setting the half-lenses 
horizontally, is the one whose use permits of the ‘ Murphy’s method ’ 
measurements, as already explained. The two half-lenses can he 
brought into symmetry with the optical axis of the spectrometer 
telescope by altering the stand itself. Finally, on a third stand 
there is arranged either a screen or a telescope eyepiece, and these 
also can be adjusted, both as regards height and position. Of 
course, in the finished model these parts are all to be collected, with 
the necessary means of adjustment, in a tube which will be a 
prolongation of the telescope tube of the spectrometer. This 
necessitates more numerous adjustments, as each of the parts has 
no longer independent motion, as when on its own stand. For 
example, means must be provided for moving each half-lens both 
in a vertical and horizontal direction by slow-motion screws. 
I am at present designing a more finished apparatus, and I hope 
soon to be able to give some of the results obtained by its use. I 
have to express my obligation to the Trustees of the Moray Eesearch 
Fund, who have provided the sum necessary for the purchase of 
part of the apparatus for the research about to be undertaken. I 
have also to express my thanks to Professor MacGregor for the use 
of the Physical Laboratory and for apparatus, and to acknowledge 
my indebtedness to him for his advice and encouragement. 
{Issued separately August 28 , 1903 .) 
