March 17. 



1005.] 



SCIENCE. 



413 



ocular, which is at right angles to the axis 

 of the objective. 



In practise the observation is made by- 

 bisecting the star, then quickly reversing by 

 turning the objective with the micrometer 

 attached through 180°, then making a sec- 

 ond bisection. One half the difference of 

 the micrometer readings w^ill evidently be 

 the measure of the star's zenith distance. 

 Obviously both bisections can not be made 

 with the star on the meridian. This makes 

 necessary a small correction easily deter- 

 mined. 



An instrument involving these principles 

 has recently been installed at the Flower 

 Observatory. So far as I am aware this 

 is the second to be constructed. In detail, 

 it differs in a mimber of particulars from 

 the Greenwich instrument. The optical 

 parts are by Brashear, and the instrumental 

 parts by Warner and Swasey. The aper- 

 ture of the objective is eight inches, the 

 focal length one hundred inches. In the 

 Greenwich instrument this cone of light 

 after reflection passes a second time 

 through the glass of the objective. In 

 this case a hole one and one half inches in 

 diameter is bored through the objective ; 

 through this hole a short tube passes, at- 

 tached above to the micrometer box. When 

 not in use this tube is closed by a shutter 

 which presses up against its lower end, 

 thus protecting the reticule from dust and 

 moisture as completely as in the ordinary 

 form of telescope. The construction may 

 be likened to that of an ordinary telescope 

 with the tube cut in two near the ocular, 

 this end being passed through a hole 

 through the middle of the objective. 



Another matter of importance is this. 

 It is evident that unless the plane of the 

 reticule passes through one of the principal 

 points of the objective, any change in the 

 inclination of the apparatus will shift the 

 zero point of the micrometer with respect 

 to the vertical. This makes necessary a cor- 



rection depending on the level readings — 

 the very thing which we wish to avoid. For 

 this purpose Dr. Hastings, Avho computed 

 the curves of this objective, so designed it 

 as to bring the first principal point in 

 front of the upper surface, 0.155 inch. It 

 was a simple matter to place the plane of 

 the reticule at this same distance. These 

 peculiarities introduced into the problem 

 some technical and mechanical difficulties, 

 all of which were successfully overcome, 

 the optical performance being entirely 

 satisfactory. 



A solid cast-iron pillar, weighing several 

 hundred pounds, formed the tube of the 

 telescope. The focal adjustment is made 

 by raising or lowering the mercury sur- 

 face, this arrangement offering no diffi- 

 culties. 



As is usually the ease with a new design, 

 in part experimental, various unforeseen 

 delays have occurred. All previous diffi- 

 culties have apparently been overcome, 

 and regular observations are now in prog- 

 ress. At present observations are carried 

 on simultaneously with this instrument and 

 the zenith telescope ; four groups are em- 

 ployed as heretofore. In this program 

 each group contains eight zenith stars to 

 be observed with the reflex tube, and ten 

 latitude pairs for the zenith telescope, with 

 one wide pair for temperature investiga- 

 tion ; the time required for these nineteen 

 observations being on the average approxi- 

 mately two and one half hours. It is 

 hoped that in the course of two or three 

 years these observations may furnish data 

 tending to throw light on a number of ob- 

 scure problems. 



The star y Cygni, magnitude 2.5. cul- 

 minates within less than one minute of 

 the zenith of the instrument. Although 

 at present it differs in right ascension from 

 the Sim by only 1^ 30™, it is an easy ob- 

 ject to observe. There will be no diffi- 

 culty in following it during the greater 



