(16) 



TRANSIT OP VENUS, 1874. APPENDIX. 



the curtate triangle in its whole height, as nearly as possible. These " trammels," as we called 

 them, are made of sound hard oak wood, with the utmost care that carpenters could give. Definite 

 corresponding points were adopted as those which represent very approximately on both trammels 

 the interval 12 inches. With trammel (1) all the intervals between the plates were measured at 

 both their extremities ; and with trammel (2) the breadth of every plate was measured at the 

 same parts. Some numerical differences were thus ascertained, totally insensible in the further use 

 of the scale. 



The scale, thus verified, was fixed above the north balustrades on the top of the Octagon Room 

 of the Royal Observatory. By permission of the Governor of the Royal Naval College, a photo- 

 graph-observatory was erected in the court of the Naval School, at a distance from the Royal 

 Observatory (perhaps 1,500 feet) which permitted the image of the scale to occupy the greater 

 part of tiie photographic plate when inserted in each of the photoheliographs. To this photographic 

 hut was brought, in succession, each of the five heliographs used in the Transit Expedition ; and 

 images of the scale were taken, in exactly the same way as the images of the Sun with Venus in 

 the Transit. The photographs were then inserted in the micrometer-apparatus, and were measured, 

 for both edges of each plate of the scale. The further treatment of these measures will be best 

 understood from the inclosed specimen of Form 22. The words and figures in italic type are 

 copied from a real observation. It is to be remarked that five revolutions of the micrometer 

 correspond very nearly to one millimetre. 



It will be seen here that the amount of distortion is considerable. Observations were made 

 with the tubes of the photoheliographs turned in different positions or "azimuths" round the 

 optical axis of the telescopes, but no certain difference of distortion could be ascertained. 



The total number of sheets of Form 22 is about 300. The numbers (for each instrument) for 

 " Excess of De la Rue's Scale" were collected from the different sheets, the means for (16) (1), 

 (15) (2), &c., were taken, and the results were laid down in separate curves for the five 

 instruments. These curves were smoothed down, and their ordinates were used to give corrections 

 for the measures of the Transit Photographs. 



The following are the Corrections thus obtained by Major Tupman, applicable to the Readings 

 in the several series of Photographs : 



