Ohjeet-glasses and their Definition. 17 



This is ominous of the series of errors which the author afterwards 

 brings forward. Anyone that has worked practically at the subject 

 will admit that the focal point of an object-glass of large aperture 

 is confined within such a very minute hmit as to distance, that it is 

 impossible to assume its position for the purpose of tracing the coui-se 

 of a ray through the entire combination. In fact, the work must be 

 commenced from the posterior or conjugate focus, which, though a 

 variable distance according to the power of the eye-piece or its 

 position, is in some cases so far back that rays only slightly con- 

 vergent or nearly parallel may be commenced with, and the result 

 will approach near to accuracy in the focus, or final convergence 

 beyond the front. 



The same optical law that limits the aperture of any object-glass 

 to near 82^ in a balsam -mounted object also determines the angle 

 in the lens at which the rays diverge after being refracted from the 

 plane surface of the front. This can never exceed 82° in a dry- 

 objective ; nor can it be greater on the immersion system, where an 

 interchange of front adapts it to both conditions, as the very correc- 

 tion which necessitates the form of the back lenses and their dia- 

 meters will not transmit a greater pencil ; and therefore if the front 

 is immersed in balsam for the purpose of viewing an object placed 

 therein, this angle of 82° or less, as the case may be, instead of 

 converging at 170° as fi'om the dry lens, is continued right to the 

 object, supposing the refractive index of the front and balsam to be 

 the same, which they are nearly. 



In Dr. Pigott's diagrams, p. 134, Plate LX., of the September 

 No. of this Journal, Fig. 2 is a form of achromatic that is now never 

 employed as a front. The focal point in each example is taken in 

 quite an impossible position, for the rays there shown arrive at the 

 convex back of the lens nearly as a radius, or from its centre of 

 curvature, so that at this surface there would be scarcely any refrac- 

 tion, the rays passing through nearly straight, most of them never 

 reaching the succeeding lens at all, and those that finally emerge, 

 instead of forming a conjugate focus behind, would be divergent ; and 

 yet after quoting my objections he still complacently refers to his 

 Plate LX. as the standard for illustrating " the aberrations of the 

 two systems." * 



The annexed diagram, Fig. 1, 28^ f times the size of the ori- 

 ginal, shows the marginal rays of mean refraction projected in the 

 exact position that they occupy through the |th described in my 

 paper " On the Construction of Object-glasses." | The focal point 

 from the j5:ont lens is '0126, or near ^Vfli of an inch, the extreme 



* See p. 254, Nov., 1870. 



t The original diagram was much larger, but was required to be reduced to 

 bring it within compass for these pages. 



X 'Microscopical Journal,' Feb. 1869, p. 112. "• • 



c 2 



