4 io THE POPULAR SCIENCE MONTHLY. 



for its use to a certain extent. The great advantage of this method is 

 that it makes it possible to perform the necessary measurements, upon 

 whose accuracy everything depends, at leisure after the transit, with- 

 out hurry, and with all possible precautions. The field-work consists 

 merely in obtaining as many and as good pictures as possible. The 

 only objection to the method lies in the difficulty of obtaining good 

 pictures, i. e., pictures free from distortion, and so distinct and sharp 

 as to bear high magnifying power in the microscopic apparatus used 

 for their measurement. It is necessary also that the exact scale of 

 the pictures, or the number of seconds of arc to the linear inch, be 

 known, as well as the precise Greenwich time at which each picture 

 is taken, and it is also extremely desirable that the orientation of the 

 picture should be accurately determined, that is, the north and south, 

 east and west points of the solar image on the finished plate. There 

 has been a good deal of anxiety lest the image, however accurate and 

 sharp when first produced, should alter in course of time through the 

 -contraction of the collodion film on the glass plate, but the experiments 

 of Rutherfurd, Huggins, and Paschen, seem to show that this danger 

 is imaginary; that if a plate is properly prepared the collodion film 

 never creeps at all, but remains firmly attached to the glass. It re- 

 quires but a very trifling amount of distortion or inaccuracy of the 

 image to render it useless. The uncertainty in our present knowledge 

 of the sun's parallax is so small that it would only involve an error of 

 about one-quarter of a second in the calculated position of Venus on 

 the sun's disk as seen from any station at any given time during the 

 transit, and this would be about ao * 00 of an inch on a four-inch picture 

 of the sun. Unless, then, the picture is so distinct and free from dis- 

 tortion that the relative positions of Venus and the sun's centre can 

 be determined from it within it) i 00 of an inch, it is worthless as a means 

 of correcting the received determination of the parallax. 



But it is to be noted that any mere enlargement or diminution of 

 the diameter of sun or planet will do no harm, provided it is alike all 

 around the circumference of the disk, since the measurement is not 

 from the edge of Venus to the edge of the sun, but between their 

 centres. Photographic determinations of contact, on the contrary 

 (such as Janssen and some of the English parties attempted by a pe- 

 culiar and complicated apparatus), are affected with all the uncertain- 

 ties of the old-fashioned observations of the eye alone, and with oth- 

 ers in addition; so that, astronomically considered, they are entirely 

 worthless, although interesting from a chemical and physical point of 

 view. 



Two essentially different lines of proceeding were adopted, at the 

 last transit, in the photographic observations. The English and Ger- 

 mans attached a camera to the eye-end of an ordinary telescope, which 

 was pointed directly at the sun ; the image formed at the focus of the 

 telescope was enlarged to the proper size by a combination of lenses 



