TRANSACTIONS OF THE SECTIONS. 3 



ments ; and I am also having constructed one of my own for this purpose and for 

 future solar observations. All these instruments, made precisely alike, will embody 

 the results of our experience gained during the last ten years in photoheliography at 

 the Kew Observatory whilst belonging to this Association. Oueonly of them, namely 

 the photoheliograph which has been at work for some years at Wilna, is of a some- 

 what older pattern ; but how great an advance even this instrument is on the ori- 

 ginal at Kew is proved by the delightful definition of the most delicate markings 

 of the sun in the pictures which have reached this country from Wilna. 



Hitherto sun-pictures have been taken on wet collodion ; but a question has been 

 raised whether it would not be better to use diy plates. On this point M. Struve 

 informs us that in two places (at Wilna, under the direction of Colonel Smvsloff, 

 and at Bothkamp, in Holstein, under Dr. Vogel) they have perfectly succeeded in 

 taking instantaneous photographs of the sua with dry plates. 



As far, however, as my own experience has gone, I still believe that the wet 

 collodion is preferable to the dry for such observations. 



Now, with reference to contact observations, which it must be remembered are 

 by no means indispensable as far as photography is concerned, it may be conceded 

 that there will attach to the record of the internal contact a certain amount of 

 uncertainty, although not so great as that which afl'ects optical observation. The 

 photograph which first shows contact may possibly not be that taken when the 

 thread of light between Venus and the sun's disk is first completed, but the first 

 taken after it has become thick enough to be shown on the plate ; and this 

 thickness is somewhat dependent on incidental circumstances — for example, a 

 haziness of the sky, which, although almost imperceptible, yet diminishes the 

 actinic brilliancy of the sun, and might render the photographic image of the small 

 extent of the limb which is concerned in the phenomenon too faint for futm'e mea- 

 surements. On the other hand, having a series of photographs of the sun with Venus 

 on the disk, we can, with a suitable micrometer (such as I contrived for measuring 

 the eclipse-pictures of 1860, and which since then has been in continuous use in 

 measuring the Kew solar photograms*), fix the position of the centre of each body 

 with great precision. But the reduction of the measured distances of the centre to 

 their values in arc is not Avithout difficulty. Irradiation may possibly enlarge the 

 diameter of the sun in photographic pictures, and it may diminish the size of the 

 disk of a planet crossing the sun, as is the case with eye-observations ; but if the 

 images depicted are nearly of the same size at all stations whose results are to be 

 included in any set of discussions, then the ratio of the diameters of Venus and 

 the sun will be the same in all the plates, and it will be safe to assume that they 

 are equally aflected by irradiation. The advantage which, therefore, will result by 

 emploving no less than eight instruments precisely alike, as are those now being 

 made by Mr. Dallmeyer on the improved Kew model, is quite obvious. If other 

 forms of instruments, such as will hereafter be alluded to, be used, it will be es- 

 sential that a sufficient number of them be employed in selected localities to give 

 also connected sets for discussion. 



To give some idea of the relative apparent magnitudes of the sun and Venus, I 

 may mention that at the epoch of the transit of 1874 the solar disk would, in the 

 Kew photoheliograph, have a semidiameter of 19(3o-8 thousandths of an inch, or 

 nearly two inches ; Venus a semidiameter of 63'33 of these units ; and the parallax 

 of Venus referred to the sun would be represented b_y 47-85 such units, the maximum • 

 possible displacement being 9o-7 units or nearly ^ of an inch. 



When the photographs have been secured, the microraetric measurements which 

 wJl have to be performed consist in the determination of the sun's semidiameter 

 in units of the scale of the micrometer, the angle of position of the successive situa- 

 tions of the planet on the disk, as shown on the series of photogi-aphs, and finally 

 the distances of the centres of the planet and the sun. These data determine abso- 

 lutely the chord along which the transit has been observed to within 0"'l ; and an 

 error of 1" in the measurement would give an en'or of only 0"-185 in the deduced 



* In this micrometer, which is capable of giving radial distances, angles of position, and 

 also rectangular coordinates, the accuracy of linear measurements does not depend on the 

 doubtful results given by a long run of a micrometer screw. 



1* 



