PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
53 
avoid back-lash, we must move the micrometer head always in the one direction. 
The adjustment cannot, therefore, be made by tentative movements, and, in 
consequence, it is likely to be erroneous. Again, we have to remove the eye for the 
first reading. Hence, if there is parallax error, we may come in for it. The 
following method has been found much better : The image of the halo is brought 
into tangential contact with one of the fine fixed lines of the eyepiece. This is 
effected to a nicety by placing the halo a little excentrically in the field and rotating 
the stage of the microscope. Next, the travelling wire is brought into tangential 
contact with the other limb of the halo. This adjustment is effected by successive 
trials, always withdrawing the line, and again bringing it up against the halo. 
Finally, leaving the micrometer at this setting, we investigate the fit of the halo 
between the lines by rotating back and forward the stage of the microscope. We 
also investigate the effects of slight changes in the focus. If all is satisfactory, we 
lastly read the amount of rotation of the head which is requisite, in order to bring 
the two lines into juxtaposition. Superimposition of the lines will read some 
constant even number determined in the setting of the head of the instrument. 
Juxtaposition involves a deduction of about one division of the head. But when the 
measured diameter of a nucleus has to be deducted—as is generally the case—the 
correction for the width of the line is automatically made if the readings for 
superimposition are adhered to throughout. 
In general, the chief difficulty is in placing the lines truly tangential to the 
halo-image. The difficulty increases with the faintness of the halo, and the faint 
early haloes are those of most interest, as will presently be seen. But with good 
lighting, and after shading the eye for some time, very consistent readings can be 
obtained. The doubtfulness of the readings does not amount in favourable cases to 
as much as two divisions of the micrometer head. Now, with the optical conditions 
referred to above, 113 divisions of the head correspond to a travel of the line of 
0'05 mm. in the field of the microscope. One division, therefore, corresponds to 
0'00044 mm. in the field. Hence those smaller haloes, which read about 0'022 mm. 
in diameter, should be measured upon each reading to an accuracy of about 4 per 
cent. One point I would specially call attention to : the advantage attending the 
use of the rotating stage of the microscope when testing the adjustment of the lines 
upon the image. This is greatly due to the increased sensitiveness of vision 
obtained by the mere movement of the image. It is doubtless a question of 
transferring the image to an uiifatigued part of the retina. I have long been 
familiar with this phenomenon, and I believe it is known to many microscopists. 
Theoretical Views as to the Formation of Haloes. 
The halo is the result of the ionising effects of the a-rays proceeding from the 
central nucleus. By a fundamental law of radioactivity, an equal number of a-rays 
is emitted by each contributory element. We assume that the nucleus is small in 
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