336 Profs. Liveing and Dewar, On the use of [April 30, 



(2) On the vascular system of Pelophilus, (Boa) madacas- 

 caiHensis. By Dr H. Gadow. 



The author made a communication on the vascular system of a 

 large specimen of Madagascar Boa. He chiefly discussed certain 

 points in the veinous system. These require further investigation 

 which he has in hand, and the results of which he hopes to 

 communicate to the Society in a more complete form. 



April 30, 1883. 



Mr Glaisher, President, in the Chair. 



Mr R. H. Solly was elected an Associate of the Society. 



The following communications were made to the Society : 



(1) On the use of a collimatinr/ eye-piece in spectroscopy. By 

 Professors Liveing and Dewar. 



We seem to owe to von Littrow the first suggestion for a 

 spectroscope with only one telescope which should serve at once 

 both as collimator and observing telescope ; but hitherto for the 

 most part such instruments have been abandoned on account of 

 the embarrassment caused by reflexions from the surfaces of the 

 object-glass. Professor Brackett (American Journal of Science, 

 July, 1882) mentions such an instrument constructed for the 

 College at Princeton, and says that it was necessary to put a 

 patch on the object-glass to intercept reflected light, although 

 it had been specially constructed with curvatures calculated to 

 reduce the inconvenient reflexions to a minimum. 



Professor Mendenhall, in the Memoirs of the Science depart- 

 ment of the University of Tokio, Japan, has called attention to 

 the advantages of a collimating eye-piece in grating spectroscopes. 



The most obvious of these are (1) the facility with which a 

 reflecting grating can be adjusted so that its plane may be per- 

 pendicular to the axis of the collimator if it is to be used in that 

 position ; and (2) the facility with which the distance of the slit 

 from the object-glass of the collimator may be adjusted so that the 

 light may be incident on the grating in a parallel beam. This 

 condition has to be fulfilled in order to obtain the best definition 

 whenever the telescope and collimator are not symmetrically placed 

 with reference to the grating, that is to say, if a reflecting grating 

 is used, in all ordinary cases. He points out however that when 

 the axes of the telescope and collimator are equally inclined to the 

 normal plane of the grating, which corresponds to the case of a 



