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V, — On the Measurement of the Angular Apertures of Object- 

 glasses. By Jabez Hogg, Surgeon to the Eoyal Westminster 

 Oplithalmic Hospital, F.K.M.S., &c. 



I AM led to infer from what has recently transpired that consider- 

 able misapprehension prevails with regard to the measurement of 

 the angular aperture of object-glasses. An article contributed by 

 Mr. Ingpen to the Journal of the Quekett Club, and quoted in the 

 ' M. M. J.,' * though of some interest, only very briefly alludes to 

 the various methods employed during the last quarter of a century, 

 and does not touch upon any method that would be considered 

 valid in the determination of the apertures of immersion lenses. 

 Mr. Ingpen makes particular mention of Professor Eobinson's 

 method, and though he speaks of it as " a very elegant method, and 

 likely to be valuable, in certain disputed cases, as to the true angle 

 of immersion lenses," it would appear that he thinks immersion 

 apertures can be measured by this method without any modifica- 

 tion. It is, however, quite evident that Professor Eobinson had 

 solely in view the measuring of the apertures of dry lenses — such 

 as have a true air-focus ; and Mr. Ingpen offers no evidence what- 

 ever that he realizes what modification is necessary in Professor 

 Eobinson's method to render it applicable to immersion lenses. 



In the Journal,! Mr. Wenham unhesitatingly affirms that Pro- 

 fessor Eobinson's method is by far the best. These are his words : 

 " I consider that the most correct of all [methods of measuring 

 apertures] is that proposed by Professor Eobinson, which consists 

 in passing the parallel rays of the sun through the back of the 

 objective, and then by means of a white screen in a dark room 

 intercepting the rays as a disk of light. The angle taken from the 

 diameter of this to the focal point will give the true aperture." 

 Mr. Wenham also appears to think that the aperture of immersion 

 lenses can be accurately determined by the same method, and 

 without modification of any kind. On the other hand, it is held 

 by competent authorities who have given the subject special atten- 

 tion, that the only possible way of measuring immersion apertures 

 with accuracy is to measure the angle of the cone of rays while 

 thev are in the condition of immersion. This view is entertained 

 by Dr. Woodward, Professor Keith, Professor Abbe, Messrs. Hart- 

 nack and Prazmowski, Messrs. Powell and Lealand, Mr. Dallmeyer, 

 Mr. Tolles, and Professor G. G. Stokes, whose authoritative utter- 

 ances on the subject are of the highest importance. 



The modification required in Professor Eobinson's method to 

 enable anyone to measure immersion apeiiures is an extremely simple 

 one, and can be readily put into practice. In place of the white 

 * Page 236, May 1876. t ' M. M. J.,' vol. viii., p. 233. 



