M.-P.-Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 65 



of observation should not exceed one-tenth of a second of 

 arc. And the double-star observer certainly expects his 

 measures of distance to differ little from that amount. In 

 the telescope the diffraction rings of the stars are beauti- 

 fully delicate. On a quiet night the stars appear, to the 

 normal eye, fixed on the sky like minute dots of light, with- 

 out a sign of twinkling or irradiation, and the intensity of 

 the light of each star, large or small, is continuous. 



The observer knows that in an atmosphere which is 

 remarkably unsteady, when every star to the naked eye is 

 dancing madly, the smallest stars have disappeared, the 

 smaller ones appear and disappear spasmodically, and the 

 larger ones near the horizon rapidly change color, and 

 even the planets have much irradiation and are blinking; 

 the star in the instrument has lost its diffraction rings, the 

 spurious disc is broken up, and the confused mass jumps 

 wildly across the micrometer thread and frequently expands 

 into a nebulous film many seconds in diameter. We have 

 measured such an apparition of Polaris forty-seven seconds 

 in diameter. 



" Burnham has remarked that an object-glass of six 

 inches will one night show the companion of Sirius per- 

 fectly; on the next night, just as good in every respect, 

 so far as one can tell with the unaided eye, the largest 

 telescope in the world will show no more trace of the small 

 star than if it had been blotted out of existence." (Webb's 

 " Celestial Objects.") 



With a serene atmosphere the transit observer is certain 

 of his times. With a disturbed atmosphere he can make 

 only an unsatisfactory estimate by eye and ear, and a still 

 less reliable one by the chronograph. 



In the course of fifty-four years' experience as an 

 observer, largely in the field, we have encountered a range 

 of physical conditions that falls to the lot of very few. We 

 have made astronomical and geodetic observations in all 

 climates, at all seasons, from the low ocean coast to eleva- 

 tions reaching 12,566 feet, and with instruments of precision 

 of various character. Our latest astronomical observations 



