ASTRONOMY. 29 



the size. He gives the considerations which indicate in 

 what respect important advances in the art of instrument- 

 making are to be looked for. First, Mr. Grnbb says that 

 beyond an aperture of 35.435 inches reflectors will have the 

 advantage over refractors. Refractors have the advantage 

 in their "greater permanence of collimation, and consequent 

 suitability for ordinary observatory work, and for measur- 

 ing purposes," as also in permanence of the optical parts, in 

 the fact that they have no central mirror to disturb the 

 course of the rays, and the comparatively slight effect of 

 air-currents upon them, since the tubes can be closed at 

 both ends. The advantages of reflectors are absence of a 

 secondary spectrum, better applicability for work in celes- 

 tial photography, photometry, spectroscopy, etc. ; the pos- 

 sibility of supporting them with perfect freedom from 

 flexure irrespective of size (perhaps this conclusion of Mr. 

 Grubb will not be generally accepted), and their general 

 convenience for observing purposes. Mr. Grubb looks with 

 most hope to large metal specula for the best results. 



The question of the relative goodness of reflectors and 

 refractors has some light thrown upon it by recent obser- 

 vations of the satellites of Mars. These were both easily 

 seen and measured in a refractor of 12 inches aperture. 

 The outer satellite has been certainly seen with a refractor 

 of 7 inches. With an 18-inch silvered-glass reflector, Key, 

 of Hereford, was barely able to see the outer satellite when 

 its exact position was known. At Marseilles the outer sat- 

 ellite has been observed with a refractor, but neither of the 

 two has been seen with the reflector of 31 inches aperture, 

 " on account of diffused lteht in the field." The silverinc: 

 of this mirror has, however, deteriorated through age. 

 With the 6-foot reflector of Lord Rosse the outer satellite 

 alone was seen up to September 20, and " not well enough 

 to measure it." At Melbourne neither satellite was seen 

 with Mr. Grubb's 4-foot reflector, and we have no account 

 of observations with Mr. Lassell's 2-foot reflector. As far 

 as has been reported, not a single reflector has even seen the 

 inner satellite. 



At the same time Grubb also publishes his new illustrated 

 catalogue of instruments, domes, etc., which is really an im- 

 portant addition to the literature of the subject. The cata- 



