30 , Transactions of the Society. 



will make the true position of the index-pointers uncertain, but 

 will add nothing substantially to the reading of aperture. 



The size of the field has practically no influence by the method 

 in question. 



As to the observation on the oil-immersion |, to which Mr. 

 Wenham alludes in j)roof of his assertion, its fallacy will be obvious 

 on a moment's reflection. When a microscopist observes any object 

 of say 7 mm. in diameter, and wants to observe its whole extension 

 within one field of vision, he certainly will take care that the stop 

 in his eye-piece does not confine the field to 6, or 4, or 3 mm. Now 

 the telescopic image of distant objects, delineated on the back of an 

 oil-immersion ^, must extend to upwards of 7 mm., as is evident 

 at once to everybody from the diameter of the back lens ; and for 

 observing the limits of this telescopic image the whole must, of 

 course, be within the field of the auxiliary Microscope. Why did 

 Mr. Wenham expect then to see the limits of the aperture whilst 

 using eye-pieces, the stops of which confine the field of vision of 

 the auxiliary Microscope to perhaps 3, or 4, or 6 mm. ? 



A few words on the origin of the apertometer. Dr. Woodward, 

 in his paper " Description of a New Apertometer," * speaks of my 

 arrangement and every part of my method as a " modification " of 

 an apparatus described by Mr. E. B. Tolles in 1873. I am 

 aware of Mr. Tolles' priority in the description of the semicircular 

 glass disk, and I highly appreciate his merits in the propagation 

 of sound ideas about the aperture subject; but Dr. Woodward 

 will allow me to observe that two principal features of my arrange- 

 ment, the observation of the telescopic field of vision of an objec- 

 tive and the numerical indication of the aperture, have obviously 

 no connection at all with a glass disk. In fact, I have applied this 

 method of observation with the naked eye and with an auxiliary 

 Microscope, since 1870, in measuring air angles by means of a 

 divided rule fixed below the stage of a vertical stand at a definite 

 distance (100 mm.) from the focal point of the objective, black 

 disks moved along this rule being used as indicators for marking 

 the limits of the telescopic field on the scale ; an arrangement which 

 I use even now with objectives of moderate air angle, the rule 

 being divided after the numerical scale. Shortly afterwards, when 

 I felt the necessity of extending measurement to apertures ap- 

 proaching or exceeding the maximum air angle, I interposed a 

 semicircular lens of well-known refractive index, centred in the 

 stage-hole of the stand, between the objective and the scale, in 

 order to prevent the angular extension of the cone of rays in its 

 passage to air and the total reflexion of the oblique pencils — a 

 device essentially identical with the arrangement described and 

 * -Am. Quart. Micr. Journ.,' i. (1879) p. 284. 



