Examination of Photographic Lenses at Kew. 409 



5. Number of Reflecting Surfaces. 



In most cases the number of reflecting surfaces of glass is known 

 at once from the type of lens, but, if in doubt, a simple experiment 

 will settle the point ; the room is darkened, and the reflection of a 

 lamp is observed in the lenses ; each of the surfaces of the lenses 

 will give one direct reflected image, and the number can thus easily 

 be counted. The amount of light which reaches the photographic 

 plate decreases with an increased number of lenses, because of this 

 reflection, and this circumstance should not be forgotten in estimating 

 the suitability of a lens for any special purpose. Surfaces merely 

 separated by Canada balsam reflect little light, and need not be con- 

 sidered from this point of view, 



6. Centering in Mount. 



Two different errors might be described under this heading : either 

 (1) the optical axis of a perfect lens may not coincide with the axis 

 of the mounting, or (2) the axes of the different lenses of a doublet 

 or triplet may not all be in the same straight line. As to the first of 

 these errors, we believe it would never be sufficient to have any 

 appreciable effect on the practical value of a lens, and therefore no 

 test for it is considered necessary. With regard to the second error, 

 Wollaston's test is the only one applied ; this consists of looking at 

 the flame of a lamp or candle through a compound lens, and noting if 

 all the different images of the light as seen by successive reflections 

 from the surfaces of the glass can be brought into line by a suitable 

 movement of the whole lens, which should be the case if the com- 

 ponent lenses are arranged about a common axis. 



It may be remarked that the nodal points may be shifted away from 

 the mechanical axis of the lens in consequence of either of the above- 

 mentioned errors, and also, on the other hand, that the second error 

 may exist that the axes of the component lenses of a doublet may 

 not be coincident and yet one or both of the nodal points may 

 conceivably be found on the mechanical axis of the mounting; it 

 follows, therefore, that to estimate the distance between the nodal 

 points and the mechanical axis, which has been suggested as a means 

 of detecting any want of centering, does not answer that purpose very 

 well. 



7. Visible Defects, such as Stria?, Veins, Feathers, fyc. 



Under this heading any faults detected by a careful inspection are 

 given. 



8. Flare Spot. 



The defect known as flare spot consists of a bright spot or patch of 

 light being formed in the centre of the field. To detect it, the lens is 



