Age of Pleocliroic Haloes. 649 



within it. It would seem that a general allowance of 50 per 

 cent, would not be too much to make; that is, we may 

 consider that only half the rays actually emitted by the 

 nucleus are effective in generating that part of the small 

 haloes which enters into the observations. 



The measurements on the dimensions of the nuclei of the 

 haloes were made in the Iveagh Geological Laboratory of 

 Trinity College, Dublin. As it is obviously very necessary 

 to secure accuracy in these measurements, care was taken by 

 attention to the best conditions of illumination and magnifi- 

 cation and by repetition of the readings to guard against 

 error. Professor H. H. Dixon, F.R.S., independently 

 checked many of the observations. It was found that the 

 best definition was obtained with an oil-immersion of Zeiss 

 (apochromatic 3 mm. N.A. 1'4), and with a No. 7 of Leitz; 

 but the oil-immersion gave the best definition. With careful 

 attention to the lighting the definition could be got very 

 perfect^ and there was no uncertainty whatever as to the 

 reliable nature of the measurements, which could be repeated 

 again and again with always the same results. The readings 

 were effected by two distinct methods : (1) by a micrometer 

 eyepiece with travelling line ; (2) by the camera lucida 

 combined with the following method of measuring the 

 diameter of the referred image. Two fine lines are drawn 

 with a drawing-pen in indian-ink on smooth white paper. 

 The lines meet at a point and very slowly diverge. The 

 separation of the lines was about 3 millimetres at a distance 

 of 15 cm. from the point of intersection. The camera 

 lucida being in position the image is referred in the usual 

 manner to the sheet of paper. The latter is now shifted till 

 the image of the nucleus appears to fit accurately between 

 the lines. While still looking at the object a mark is made 

 with a pencil across the lines just where the nucleus is 

 referred. This is, say, at a distance d x from the intersection. 

 An engraved scale divided to O'Ol mm. is now substituted 

 for the mica, and one of the subdivisions brought to fit, as 

 before, between the lines, at a distance d 2 from the inter- 

 section. This point is marked. The diameter, x, of the 

 nucleus is then obviously found from the ratio x : O'Ol : : d 1 

 : d 2 . This method gave very consistent results in successive 

 measurements *. 



In some few cases the diameter of the nucleus is the mean 



of two not very different diameters, the development of the 



nucleus not being such as would permit of the assumption 



that it was of approximately spherical form. In some cases 



* Proc. E. D. S. xiii. p. 441. 



