41(5 Major L. Darwin. On the Method of 



FIG. 3. 



and an adjustment is necessary ; this is done by moving the lens- 

 holder in or out, thus making the axis of rotation pass through different 

 parts of the axis of the lens, until the image ceases to show any 

 movement ; and this can only be the case when the axis of rotation 

 does pass through the nodal point of emergence. As far as the above 

 considerations are concerned, it is immaterial how far off the small hole 

 in the tin plate is from the lens, but if the horizontal axis has not been 

 made to pass accurately through the nodal point of emergence, this want 

 of adjustment will have much the same effect as a small vertical move- 

 ment between the two readings of the vertical arc. It is evident that 

 the angular error thus produced will dimmish as the distance of the 

 point of observation increases ; moreover, any distortion at the edge 

 of the plate will make the above theoretical considerations no longer 

 strictly applicable, and will have the same effect as the axis of rota- 

 tion not accurately passing through the nodal point. In order, there- 

 fore, to minimise these sources of error, the tin plate with the hole in 

 it is removed as far as practicable from the lens before the observa- 

 tion is made. 



The angle of the inner cone, that is, of the cone outside which the 

 opening of the stop is partially eclipsed by the mounting of the lens, 

 &c., is measured in the same way as above described for the outer 

 cone, and with the same precautions. When looking through the 

 small hole, the positions on each side of zero at which the aperture 

 begins to be shut off, and beyond which it no longer appears as a per- 

 fect ellipse, are easily seen, and the angle between these two positions 

 as measured on the vertical arc gives the angle required. The angles 

 of these two cones are generally given when the observation is made 

 with the largest stop supplied with the lens. 



The results of these measurements should be considered in connex- 

 ion with test No. 17, under which heading the general question of 

 illumination of the field will be discussed. In order to facilitate 

 the consideration of the covering power of the lens, the diameters of 



