122 Transactions of the Societij. 



order to eliminate any additional movement due to error in cor- 

 rection of the objective. If the image of the illuminant moves in 

 the same direction as the slit, it signifies under-correction, and 

 a more distant light source is therefore required ; if the opposite 

 direction, over-correction is present and the light source should be 

 nearer. The rule is therefore the same as that found for an objec- 

 tive : " Movement with " equals " plus " equals " a greater distance 

 is required ; " " movement against " equals " minus " equals 

 " nearer light source should be used." 



The Theory of the Method. 



The basis of this method of adjusting tube length may be of 

 interest, and is therefore briefly described as follows : — 



In the case where the tube length has been properly adjusted 

 spherical aberration is eliminated, and therefore rays starting from 

 the point M'here the optic axis meets the specimen all meet at a 

 point again on the axis. Therefore, as different objective zones 

 are exposed the image remains stationary. 



If the tube length is too short, then, as is well known, the 

 outer rays focus closer to the objective than the inner rays — that 

 is to say, rays from the outer zones of the objective have met 

 one another to form a focus, and have then crossed over to the 

 opposite side before the plane of the focus for the more central 

 rays is reached. Those from the left-hand edge of the objective 

 aperture will therefore be found on the right, and vice versa. Now, 

 the left-hand edge of the objective aperture is exposed when the 

 slit beneath the condenser is to the right, and therefore when the 

 slit is on the right the image is also on the right, and vice versa ; in 

 other words, the image moves in the same direction as the slit. 



If the tube length is too long, then the central rays form a 

 focus before the outer rays have done so. At the plane of the 

 focus of the central rays, therefore, the outer rays have not yet met — 

 that is, the rays from the right-hand edge of the objective aperture 

 are still on the right, and those from the left on the left. Since, 

 as before, the right-hand edge of the objective aperture is exposed 

 when the slit below the condenser is to the left, the image is on 

 the right when the slit is on the left, and vice versa, and the 

 motion of the image is therefore in opposition to that of the slit. 



Relative Advantages of Different Methods. 



The three alternative methods are : — 1. Trial and error method. 

 2. Ainslie's method. 3. Author's method. 



1. The trial and error method suffers from tha disadvantages 



