ASTRONOMY. 441 



impressiug on a Morse fillet the figures and divisions of the micrometer 

 head along- with those of a fixed index. 



The instrument is now being devoted to systematic investigations in 

 stellar parallax, and work is progressing, which will furnish, it is hoped, 

 reliable values of the parallaxes of the ten stars of the 1st magnitude 

 in the Northern Hemisphere referred to neighboring stars of about the 

 8th magnitude. 



Zurich. — The sun-spots are assiduously observed here. The maximum 

 occurred in December, 1883, to January, 1884. 



ASTKONOMI(!AL INSTRUMENTS. 



Wire-gauze screens as photometers. — " Of late years the use of wire- 

 gauze screens, one or more in number, over objectives has come into 

 use for several purposes. Over one of the halves of a heliometer-objec- 

 tive they are used to reduce the image of a bright star to approximate 

 equality with that of a fainter star from the other half, an essential con- 

 dition for the most accurate superposition of the two images. With a 

 meridian circle they are used to reduce the brighter stars to an approx- 

 imate equality with the faintest that can be observed with satisfactory 

 j»recision, or to investigate tbe difierence of personal equation for dif- 

 IVrent magnitudes by taking different tallies of transit- wires, with screen 

 off and on, at the same transit. In the latter case Professor Holden 

 points out the necessity {Astroti. Nachr. 2690) of changing the illumina- 

 tion of ihe field with the change of screen, so that each magnitude may 

 show against its customary degree of color or brilliancy of background. 

 Such screens may also be used for photometric purposes when once 

 their coefficients of transmission have been determined. Those having 

 occasion to use them in this way will do well to consult a paper by Pro- 

 fessor Langley {Amer. Journ. Sc, xxx, 210) on this subject. In this it 

 is shown that the effective transmission coeflBcients are decidedly dif- 

 ferent according as the luminous image is an extended surface or i>rac- 

 tically a i)oint like a star. In the latter case there is a central image 

 surrounded by a system of diffraction images, into which a large part 

 of the light goes ; so much so, that Professor Langley found that a screen 

 (of which one and two thicknesses transmitted -47 and -21 respectively 

 of the full light upon a surface) gave only -18 and -02 for one and two 

 thicknesses, respectively, M'hen measured by the brilliancy of the cen- 

 tral image of a small i)in-hole as a source of light." (Science.) 



The defining power of telescopes. — Quite a warm discussion has been 

 going on in the columns of the Observaforij and Sidereal Messenger on 

 the relative merits of small and large telescopes for the study of plan- 

 etary detail. The controversy seems to have arisen from a statement 

 by Mr. Denning to the effect that " apertures of from 6 to 8 inches seem 

 able to compete with the most ijowerful instruments ever constructed." 

 The true state of the case appears to be given in a note by Professor 



