518 RECORD OF CURRENT RESEARCHES RELATING TO 



The next eight columns give the description of the objective as 

 obtained from the maker ; viz. its price,* designation, adjustment for 

 cover (if any), dry or immersion, equivalent focal length, and its 

 aperture in air, water, or balsam. The remaining columns show the 

 results of examination. The first group shows the extreme angle for 

 the admission of light from the centre of the field, measured in 

 air, water, or balsam ; the next group the extreme angle for best 

 definition, also measured in air, water, or balsam. This latter value 

 is usually, though not always, less than the extreme angle for admis- 

 sion of light ; and the difference between them, which is sometimes 

 very considerable, shows the extent to which the nominal aperture of 

 the lens could be profitably reduced by diaphragms as proposed by 

 Dr. Eoyston-Pigott. The next column gives the nominal amplifying 

 power, that is the amplifying power of the objective alone at 10 inches 

 from the optical centre of the objective. Providing its nominal equiva- 

 lent focal length were correctly given, this would be for a ^-inch x 20, 

 &c. The next column shows the actual amplifying power, measured 

 at 10 inches from front surface of front lens, of the objective. It 

 would have been more absolutely accurate to measure from the optical 

 centre, but the exact position of this point could not be ascertained 

 without a knowledge of the formula on which the objective was con- 

 structed, and in lenses possessing adjustment the position of the 

 optical centre varies with every movement of the adjustment. The 

 front surface of front lens being a fixed and easily determined position, 

 and the results obtained by using it differing but slightly from those 

 obtained by using the true optical centre, it was selected as, on the 

 whole, the best position from which to measure the 10 inches. The 

 method of measurement has been to compare the image of a Eogers' 

 stage micrometer with that of an eye-piece micrometer placed exactly 

 10 inches from front lens, both micrometers being magnified by means 

 of a Bausch and Lomb (Gundlach) periscopic eye-piece, in which the 

 field lens is placed within the focus of the eye lens, and the eye-piece 

 micrometer placed below the field lens and in the focus of the eye- 

 piece, as a whole. The ordinary Huyghenian eye-piece will not 

 answer for this kind of work. The actual and nominal amplifying 

 powers of objectives are often very closely identical, and where varia- 

 tions occur, the actual will sometimes be found in excess of the 

 nominal, and vice versa. The next column gives the frontal distance 

 of the objective, that is, the distance from front of front lens or of 

 brass setting to the object ; and the next column, the " working dis- 

 tance," which is frontal distance minus thickness of cover-glass (in all 

 cases here being • 002 inch, the same cover being used with all objec- 

 tives). The next column gives the clear aperture of front lens, that 

 is the diameter of the light-spot seen when the objective is reversed — 

 held with front lens towards the eye and the posterior combinations 

 pointed at a brightly illuminated white surface, such as a sheet of 



* The price is given as a matter of justice to makers whose cheap objectives 

 appear in the list, as it would be unjust to compare the work of a cheap objective 

 with one of same nominal power costing twice or three times as much, without 

 making the fact apparent. 



