Maj-er.] -cUo [Oct. 1, 1S69. 



corresponding to a time marked on the face of the clock employed; and 

 this correspondence of figure and time is unaftected with personal equa- 

 tion either of sight or of hearing. Now if the error and rate of the 

 clock can be entirely freed from the personal equation of the observer 

 who determined them, and if the longitude of the station be found by 

 the coincidences of the beats of a sidereal clock with those of a break- 

 circuit mean solar clock placed at the observatory of the first meridian, 

 we have the most accurate means of obtaining the absolute times of con- 

 tacts at the station of observation. 



Thus we see how apjilicable will be photography to these observations, 

 for the data of the solar parallax will be given either by observing the 

 absolute time 6f the ingress or of the egress (which method is alone of 

 value in the transit of 1874), or by determining the duration of the tran- 

 sit of Venus over the solar disc. The photographs are permanent phe- 

 nomena on which we can repeat our measures at leisure, with every ap- 

 pliance of precision, while it is impossible to attain a similar degree with, 

 eye and ear, from the difficulty of micrometically measuring at n, precise 

 instant the distance of Yenus frcm the sun's limb, and fi-om the (record- 

 ed) distortions of Yenus at contacts. 



It will also be of great value to have a photographic record of the 

 appearance of Yenus at the contacts, for, if the. disc of the planet then 

 should appear on the plate to depart from a circle and have attachments 

 to the sun's limb, these distortions can be measured and allowed for. 



An idea may be formed of the apparent size of Yenus during its tran- 

 sit of the sun's disc, from the fact that the umbra of the solar spot in 

 the south-west quadrant is 15" in diameter, and that Yenus at transit 

 will subtend an angle of about 70"; so that the lalanet would appear on ■ 

 the plate as a disc 4| times the diameter of this spot, or, as a disc of .107 

 inch diameter on an image of the sun of 3 inches in diameter. 



The negatives of these photographs I find from trial will stand a mag- 

 nifying power of 50 vmder the micrometer, and as 1" of arc will equal 

 •jjiT^th. inch on a solar image of 3 inches diameter, we can, with the above 

 mentioned power, divide a second into ten parts. This supposes, how- 

 ever, that the bisection by the micrometer thread is on a perfectly well 

 defined point, and this does not exist in the outlines of any photograph, 

 and especially is the limb of the sun indistinct on account of its shading, 

 and of the manner in which the silver is deposited in the collodion film. 



From actual experience in measurements under the micrometer, I find 

 that we cannot, as yet, hope to make a bisection on the sun's limb closer 

 than ^ of a second. On the boundary of the umbra of a well defined 

 solar spot, we can read to :| of a second, and fi'om this I should think the 

 •/o of a second might probably be attained as the limit in a reading on the 

 imb of the image of Yenus. 



But with measures as close as these, and the tables of Yenus brought 

 to the accuracy which existing unreduced observations can give, we may 

 reasonably hope for a determination of the solar parallax comporting 

 with the most exact astronomical measures of this century. 



The Lehigh University, Pa., 1th September, 1869. 



