114 THE TRANSIT OF VENUS AS OBSEETED AT EDEN, 



motion of the telescope. All being ready, I waited anxiously for 

 a clear niglit to enable me to make the necessary star observations 

 for time and instrumental adjustments ; but so uniisually cloudy 

 was the weather that I could get no satisfactory observations until 

 Saturday, December 5. On the 7th and 8th I exchanged longitude 

 and clock signals with the Sydney Observatory, but on each 

 occasion was prevented by clouds from getting more than one 

 transit observation. 



On the 7th, being a clear day, we took two sets of photographs 

 to satisfy ourselves that all was in good working order, and found 

 that by reducing the aperture of the telescope to three inches the 

 sun's edge was more sharply defined and the reference lines more 

 clearly distinguished. 



In our trial observations of the sun several of our dark glasses 

 were cracked by the heat ; so finding that I could get no sufficient 

 protection even with the 3-inch diaphragm, I constructed one 

 2 inches in diameter, which gave very satisfactory results. 



In consequence of the continued cloudy weather my instru- 

 mental adjustments were not so accurate as I wished. I was 

 assisted one day in adjusting the reference lines in the camera by 

 a small well defined solar spot, which appeared to traverse one of 

 the lines with great accuracy. In order to correct any remaining 

 error in the position of the lines, I adopted the plan recommended 

 by Mr. Eussell of taking two photographs of the sun, at an 

 interval of about a minute, on the same plate, and determined to 

 repeat the process at every half-hour during the transit. Now, 

 in order to make this double image of any service, it is necessary 

 that the common tangents to the two images should be exactly 

 parallel to the direction of the sun's motion. Eor this purpose 

 the telescope must remain perfectly at rest, and therefore must 

 not be touched during the interval. 



This result appeared difficult to obtain, as the flashing shutter 

 must be made to cross the field a second time for the second image. 

 The method which I contrived, though somewhat complex, appears 

 to be perfectly satisfactory. 



The flashing shutter, as described by Mr. Eussell in his paper 

 read before the Society on September 3, 1873, is attached to the 

 end of a lever, which is drawn down by an elastic band, when 

 the other end is released by pressing a spring. If the second 

 image were obtained by raising the shutter quickly by the hand 

 at the end of the desired interval the action of so raising it would 

 probably displace the telescope ; or if the dome shutter were 

 closed, or a cap placed on the telescope, and the flashing shutter 

 restored to its former position and again released, there would be 

 the same risk and almost certainty of displacement. To overcome 

 this difficulty I arranged as follows : — ¥ov distinctness I call the 

 end of the lever to which the shutter is attached A, and the 



