TRANSACTIONS OF SECTION A. 555 



hardness leads to useless expenditure of power and tools, and in such circum- 

 stances a soft metal is much to be desired. 



The apparatus is intended to be used in connection with tensile tests of the 

 metal, and in this way afifords valuable information as to the mechanical properties 

 of the material. 



5. On Star Photography. 

 By Isaac Roberts, F.B.A.8., F.G.S. 



During the past eighteen months I have been at work taking photographic 

 negatives with a twenty -inch silver on glass reflector for the purpose of mapping 

 the stars in the northern hemisphere, between the pole and the equator, and up to 

 the present time about four hundred plates, each covering four square degrees of 

 sky, inclusive of overlap, have been secured between the pole and declination fifty- 

 seven degrees. The extent of the work done would have been much greater if the 

 weather had permitted. 



The negatives are exposed for fifteen minutes, which is sufficient time to show 

 very faiat stars, the magnitudes of which have not yet been determined. Stars of 

 the ninth magnitude are photographed faintly in one second. 



Recently I received from Admiral Mouchez, the Director of the Paris Observa- 

 tory, four magnificent enlarged photographs of stars in the constellation Oygnus 

 taken with the 13-inch refractor made by MM. Henry. The negatives were 

 taken one in June and three in August last year, and I deemed it desirable to direct 

 my 20-inch reflector on to the same sky spaces, and take negatives at the corre- 

 sponding time this year, with similar duration of exposures, namely, sixty minutes 

 each, so as to enable comparisons to be made between the results obtained with 

 two instruments constructed on different principles and with unequal apertures. 



The five photographs which were exhibited are the result. Those marked Nos. 

 1 and 2 have M 21'' 2", and Declination -i- 38° 12', one being exposed for thirty-four 

 minutes, and the other for sixty minutes. No. 3 has M 19" 45", and Declination 

 + 35° 30'. No. 4 has M 19" 55'^, and DecUnation + 37° 45'. No. 5 has M 20" 4", 

 and Declination + 35° 30'. 



The enlargements have been made to correspond very nearly with MSI. Henry's 

 photographs, and on comparing them with these it will at once be observed that 

 the appearance of the star discs difiers. In the Henry photographs the discs are 

 round, with perfectly sharp circumferences, whilst the reflector shows them round, 

 but with circumferences somewhat undefined, and presenting more the diffraction 

 appearances which always accompany telescopic eye-observations of stars. 



Another very striking difi'erence is the equal brightness of the Henry star 

 discs. Fifteenth or sixteenth magnitude stars seem as bright as those of first magni- 

 tude, and difl'er from them only in the diameter of discs, whereas the reflector 

 shows gradations between the brightest and faintest stars that wiU severely tax 

 the powers of classification. They diminish till they are lost in the colour of the 

 film on the paper, and on the negatives they can be traced to still fainter degrees, 

 and the imagination finds no difficulty in following the diminution till space from 

 oui' point of view appears to be filled with stars. 



The reflector also seems to have the advantage over the refractor in the number 

 of stars photographed in a given time ; for instance, if I select at random any square 

 inch of surface upon one of the Henry's plates, and count the stars in it, they number, 

 say, fifty-nine ; but in the same space on my plate they number 109, bein^ in the 

 ratio of nearly two to one. Of course this is a rough mode of making the 

 comparison, but it is the readiest method available at present. 



The plates which I now submit are not to be considered exceptional or picked, 

 but as average samples of those I can produce on any moderately clear night with 

 the mirror film in an average state of polish. 



If the photographs numbered 1 and 2 be compared with each other the relative 

 number of the stars which were imprinted on the films with thirty-four and sixty 

 minutes respective exposures may be counted. 



A photograph of my duplex telescope and observatory at Maghull was alsO' 

 exhibited. 



