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XL IX. Intelligence and Miscellaneous Articles. 

 THE ECHELON SPECTROSCOPE. 



Blythswood Laboratory, Renfrew. 

 April 9th, 1900. 



To the Editors of the Philosophical Magazine. 



GrENTLEMEN, 



r FHEKE are one or two corrections which should be noticed in 

 -■- the paper on the Echelon Spectroscope, published in your 

 last issue, by Lord Blythswood and myself. 



On p. 395 the double-order photograph of the blue (4358) 

 mercury line, with a field of 24,000 units, appears to exhibit 

 considerable asymmetry ; this, however, is in part clue to an error 

 of the nature described on p. 392, but chiefly to the fact that, 

 for purposes of reproduction, the outer lines were artificially 

 strengthened, and this has not been done with sufficient care. In 

 no case was any measurable asymmetry observed in the dispersion 

 of the outer components of the resolved lines. 



Dr. Larmor has kindly pointed out a slip on p. 400 in the value 



° f A^ET which should be 2 ' 2 c l- P- instead of 2-3 xlO" 13 ; on 

 p. 402 the equation for the difference in frequency of the lines 

 eHv 2 . 



should be n. — w„= „ A r , i 



1 - 2-nm lours very truly, 



E. W. March ant. 



P.S. — The details of the photographs on p. 399 of the green 

 mercury line, taken with strong magnetic fields, have disappeared 

 in the reproduction. 



ON AN INTERFERENCE METHOD OF MEASURING THE DIAMETERS 

 OF DISTANT PLANETS AND STARS. BY M. MAURICE HAMY. 



In 1868, Eizeau pointed out that the observation of interference- 

 fringes might possibly furnish a method of rinding an upper limit 

 to the apparent diameters of stars. 



If a screen with two parallel slits is placed in front of the 

 object-glass of a telescope, and the latter is then directed towards 

 a celestial object, a system of interference-fringes is, as is known, 

 obtained in the focal plane. The fringes do not appear unless the 

 apparent angle of the source situated in front of the apertures is 

 sufficiently small. In fact, each point of the source produces its 

 own system of fringes, and in order to avoid complete confusion 

 of the fringes, it is necessary that the apparent angle e of the 

 source, viewed from the first nodal point of the object-glass, should 

 be less than the apparent angle of a fringe, viewed from the 

 posterior nodal point. 



This forms the basis for establishing an approximate relation 

 between the diameter e and the distance apart of the slits which 

 causes the fringes to vanish. 



