1894.] of prisms for a stellar spectroscope. 141 



The observing telescope is pointed towards and is mounted 

 so as to turn about the same axis through as that about which 

 the prisms turn ; this single motion is enough to bring every part 

 of the spectrum in turn into view. Thus the advantages of a 

 two-prism spectroscope are obtained without the disadvantages 

 arising from the usual double adjustment necessary in directing 

 the observing telescope. The combination which I describe may 

 therefore replace a grating, in a diffraction spectroscope. 



If a bright star is observed, both spectrum and pointer are 

 bright ; for a faint star, the brightness of the pointer is appropri- 

 ately subdued. The fact that the brightness of the pointer 

 maintains a suitable proportion to the brightness of the spectrum 

 to be investigated is a great convenience. 



In astronomical work, the object-glass of an equatorial is used 

 to throw an image of the star, whose spectrum is to be studied, 

 on the slit of the spectroscope. If the slit is widened, the image 

 of the star itself is seen in place of the pointer. This is a great 

 convenience in as much as in most cases the star may be thus 

 identified. When the star is recognized amongst its neighbours, 

 the slit is closed to a suitable width and the 'pointer' then 

 appears as a short narrow line in the spectrum. In practice it is 

 preferable to have the pointer not actually superposed on the 

 spectrum, but displaced so as to be a little above or below the 

 spectrum. This end may be attained by slightly tilting the 

 reflecting prism. 



February 12, 1894. 

 PnoF. T. McKenny Hughes, President, in the Chair. 

 The following Communications were made to the Society. 



(1) On a suggested case of Mimicry in the Mollusca. By 

 A. H. Cooke, M.A., King's College. 



The species concerned were Strombus mauritianus L., and 

 S. luhuanus L., the shells of which differed from those of all other 

 Strombus in their close resemblance to the shell of Gonus, a genus 

 with which they are known to live : Strombus being a frugivorous 

 animal with small and weak teeth, and Gonus on the other hand 

 being carnivorous, with very large and barbed teeth, provided with 

 a poison bag and duct. It was suggested that this resemblance 

 must tend greatly to the advantage of the Strombus, since the 

 dangerous properties of Gonus would tend to prevent its being 

 touched by predatory fishes. 



