650 Profs. J. Joly and E. Rutherford on the 



the nucleus possesses a definite crystalline form, apparently 

 dimetric. Two dimensions are then read : the axial length 

 of the prism and its width transversely. 



In comparing the density of the staining of the halo with 

 that of the artificially darkened areas, the following procedure 

 was found most satisfactory : — Two microscopes are placed 

 side by side. The halo is observed in one of them, the spot 

 in the other. The magnification is alike and the light is 

 from the same source. Daylight only could be used. The 

 spot covered but half its field, and the adjoining unacted on 

 mica was, when necessary, brought to the exact same colour 

 and luminosity as the mica around the halo by interposition 

 of a tinted solution and manipulation of the substage fittings. 

 In the case when the mica containing the halo is the lighter 

 in colour these adjustments are applied to it. The depth of 

 tint of spot and halo are now compared. These comparisons 

 were always repeated without adjustment of the colour and 

 luminosity of the field. The former were more consistent 

 and more easy to be sure of. Professor H. H. Dixon quite 

 independently checked many of these comparisons. In the 

 majority of cases the comparison was restricted to determining 

 whether the halo should be judged lighter or darker than, or 

 the same as, the spots A or C. Generally there could be no 

 doubt as to the conclusion arrived at. It will be seen 

 presently that the interest of the results principally turns on 

 that conclusion. However, an effort was made to evaluate 

 the difference in staining of the halo and the spot in a few- 

 cases. For this purpose a transparent screen for producing 

 different graded amounts of light absorption was prepared 

 as follows : — A " Process " plate was exposed in the dark 

 room in such a manner that successive strips of it received 

 quantities of light in the proportions 1:2:3:4, &c. This 

 was done by uncovering successive strips of it at intervals of 

 one second. When developed and fixed, and a slip cut from 

 the plate with a diamond, an absorption screen was available 

 having successive areas darkened approximately in the 

 proportions of the exposures. It was found that spot A was 

 almost exactly of the darkness of the 3 second exposure; this 

 area bringing the field of the spot into agreement with it. 

 From this we may assume that the area receiving 6 seconds 

 exposure is of double the density of spot A. 



The mode of using this screen is evident from what has 

 been already said. Colour and luminosity having been 

 adjusted as before so that the fields of both spot and halo are 

 alike, the screen is slipped in above the spot (or halo) till the 

 darkening of these objects is brought into equality. 



