6 SCIENCE PROGRESS 



and two 5-foot mirrors represent an even greater advance; 

 there has been also an enormous increase in sensitiveness of 

 plates. It was in this last particular that Bond failed to allow 

 sufficient play to his imagination, as instead of an increase 

 represented by one stellar magnitude we have more than ten 

 times that estimate. But Bond's discernment was otherwise 

 so great that this slight failure may be pardoned. His post- 

 script shows that he realised even thus early the accuracy 

 of the photographic method, and in this his judgment agreed 

 with that of L. M. Rutherfurd, who set to work to measure 

 his photographs systematically and soon found that they 

 recorded the positions of the stars more accurately than his 

 own apparatus would measure them. He used a micrometer 

 screw and found, though he had provided himself with the 

 best one available, that its errors were sufficiently large to 

 prevent his doing justice to the photographs, and he turned 

 aside from his original project to the construction of a better 

 screw. Ultimately he made a screw so accurate that his 

 attention was again distracted towards the completest possible 

 test of its accuracy. This he found in the ruling of very fine 

 lines close together on metal — several thousands within an 

 inch — the result being what is called a grating, which can be 

 used like a prism to spread out light into a spectrum. This 

 work was so engrossing that Rutherfurd never seriously 

 returned to his original purpose of measuring his photographs, 

 but many of them have been measured since and have shown 

 clearly how correct was his judgment of the accuracy of the 

 photographic method. In spite of this accuracy, however, 

 the inconvenience of the wet plate long delayed serious use 

 of the method for the determination of star places. Photo- 

 graphs of the sun were taken showing the spots (requiring 

 only a momentary exposure) ; measures of spot positions were 

 made on these and found satisfactory. But a sun spot is an 

 irregular object having no very definite position and does not 

 afford a very severe test of accuracy ; consequently this work 

 failed to draw the attention of astronomers to the full resources 

 at their command. 



The complete change in attitude came in a rather 

 sensational manner on the appearance of the great comet of 

 1882. This comet, which was quite a respectable object in 

 the Northern Hemisphere, was much more magnificent in the 



