LOCAL PARTICULARS OF THE TRANSIT OF VENUS, 1874. 75 



And, lastly, we have the photographic method, which is really 

 a modification of the first or direct method. Similar stations are 

 required for both ; but it is thought that as the photographic ofters 

 the best means of obtaiiiing a great number of pictures in 

 which the image of the whole sun with Venus on it will be taken, 

 and the time at which each is photographed recorded instanta- 

 neously, that the results thus obtained will be better than those 

 obtained by the micrometer, because the position of Yeuus is 

 recorded instantly in such a way that the two measures required 

 can be made at leisure ; and the whole, or aA'erage of the whole 

 edge of the sun used so as to do away with uncertainties depend- 

 ing upon distortion affecting the whole or part of it. Some even 

 go so far as to expect from this method the best results ; and there 

 can be no doubt that, if the instrumental difficulties can be over- 

 come, it is the best form of the direct method, and has the great 

 advantage of doing away with all personality or uncertainty 

 depending upon peculiarities of the observer. Very great care, 

 however, will be requisite in the preparation and management 

 of the instrument and the photographic manipulation ; and 

 although the results will be free from peculiarities of the 

 observer, they will be affected with instrumental peculiarities, 

 of which distortion is perhaps the most troublesome. By 

 this is meant a defect depending upon the form of the lenses 

 used, which will make the images of equal portions of the sun 

 occupy unequal portions of the photograph, according to the 

 part of the field of the telescope which is used ; in other 

 words, a given angular space on the sun will occupy a different 

 space on the photograph if it is at the centre of the field towhat 

 it will if near the edge of it. So that if, when one picture is 

 taken, the centre of the sun occupies the centre of the field, and 

 when another is taken it occupies another place, the two pictures 

 will not be strictly comparable unless the actual amount of dis- 

 tortion of every part of the field is known and allowed for. But 

 another great difficulty is to ensure that the exact angle of posi- 

 tion of Venus on the sun may be determined. For this purpose 

 fiOTHQ fixed object in the telescope must be photographed by its 

 shadow with eveiy picture taken ; and lastly, we notice the diffi- 

 culty of exposing with sufficient rapidity and uniformity the 

 the whole disc of the sun to the photographic plate ; for if one 

 part of the sun be exposed longer than another it will extend the 

 sun's edge in that direction, and of course vitiate the result. 

 IS'o thoroughly satisfactory method of doing this has yet been 

 published, and the first plan used seems to be the best. It is 

 that which places a flashing shutter in that point of the telescope 

 where the rays cross, and where of course the least amount of 

 motion makes the exposure. But I must defer the consideration 

 of these points till afterwards. 



