JOUENAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



DECEMBER, 1914. 



TRANSACTIONS OF THE SOCIETY. 



IX. — Notes on some Focometrie Apparatus. 

 By Fkederic J. Cheshiee. 



(Read November 18, 1914.) 

 (Figs. 52-55.) 



Telecentric Adjustment. — When the magnifying power of an 

 objective is determined in the usual way, by projecting an image 

 of the divisions of a stage-micrometer on to a scale in the eye- 

 piece, it is important that the image of the stage-micrometer be 

 focused accurately in the plane of the eye-piece scale, as, otherwise, 

 a fallacious reading will be obtained. Fig. 52 shows diagrammatic- 

 ally how this occurs. The simple lens projects an image of a 

 scale S into the plane E. Since the lens is working at full aper- 

 ture, the principal rays of the imaging pencils will all pass 

 through the centre of the lens, and diverge outwards in the image- 

 space. If the eye-piece scale, therefore, is adjusted within the 

 plane E, the length read off as that between the images of the 

 stage- micrometer divisions, will be the length between the inter- 

 sections of the principal rays with the plane of the eyepiece-scale, 

 and will therefore be too small. If the eyepiece-scale, on the 

 other hand, is adjusted outside the plane E, the length read off 

 will be too great, for a similar reason. Abbe has pointed out that 

 this source of error can be avoided in optical measuring instru- 

 ments generally, whenever it is possible to place a small aper- 

 ture in the principal focus of the lens on the object side, so as 

 to secure what he terms " telecentric adjustment " on the image 



Dec. 16th, 19U 2 M 



