142 



TIIE PRESIDENTS ADDRESS. 



Spiierical Aberration* is a term which refers to a condition 

 inherent in every uncorrected lens, by which is meant that the 

 rays passing through its periphery, or outside portion, focus on 

 the axis at a shorter distance than those passing through the 

 more central portions of the lens, as shown in Fig. 1. If by 

 suitable correction the rays are approximately brought to a 



common focus on the axis, the lens is said to be spherically 

 corrected, as in Fig. 2 ; but if they are not enough corrected, 

 although the foci are brought nearer each other than obtains 

 in the uncorrected lens (Fig. 3 for example), the condition is 

 spoken of as under-correction. In Fig. 4 we find just the 

 reverse. The whole correction is too much, because we see the 

 outer ray is taking the position occupied by the central ray, 

 and the central ray that formerly occupied by the outer. As 

 the correction is here too great, we call this state of things 

 over-correction. 



Astigmatism is a trouble which may occur in microscopical 

 objectives, but is mostly found in photographic lenses for the 



ordinary camera. As I dare say most of you are aware, the 

 presence of this defect is indicated by the fact that it is impos- 

 sible to simultaneously focus the two cross-bars of a simple +, 

 placed so that one bar points towards the optical axis, at the edge 

 of the field. When the radial line of the cross is sharp, the 

 tangential one is fluffy ; when the latter is sharp, the definition 



