THE VICTOEIAN NATURALIST. 59 



tralian Amarygmid Beetles ; new genus and species of Buprestid 

 Beetle — viz., Hypostigmodera variegata (Blackburn), Queens- 

 land. By Mr. C. French, jun. — Orchid, Dipodium punctatum, 

 flowering at Cheltenham in June ; this orchid flowers generally 

 in the hottest months. By Mr. J. Gabriel. — Large Oyster, taken 

 off Williamstown. By Mr. H. Giles. — Species of Isaria, found 

 at Gembrook, April, 1893. By Mr. H. F. Hill. — Larvae of 

 Heplalus (?) robertsii, with parasitic growth, from Auckland, N.Z. 

 After the usual cojiversazione the meeting terminated. 



THE ILLUMINATION OF MICROSCOPIC OBJECTS. 



In his preliminary remarks on the above subject, at the 

 Field Naturalists' Club practical meeting on 26th June, 

 Mr. J. Shephard pointed out that the methods intended to 

 be illustrated had only lately been put forward in a form 

 accessible to microscopists, and emphasis was laid upon the 

 fact that once mastered they would do away with the uncertain 

 results, of the common haphazard way of " twisting about the 

 mirror until the whole field was bright," would give superior 

 results, and would save valuable time. By means of diagrams 

 drawn on the blackboard, Mr. Shephard explained the meanings 

 of the terms — spherical aberration, chromatic aberration, 

 achromatism, angular aperture, numerical aperture (N.A. as 

 usually written), and critical image, defining the latter in 

 Dallinger's words as " an image in which sharp, clear, bright 

 definition is given throughout, free from all rottenness of outline or 

 detail." The conditions of perfect illumination were then stated 

 in these words — "that with transmitted light the object should 

 be at the apices of two equal cones of light formed by the rays 

 converging upon the object and diverging again to the limits of 

 the objective," and it was pointed out that imperfections in even 

 the best lenses necessitated some reduction in the illuminating 

 cone. Touching the use of the concave mirror, the necessity of 

 focussing it, and of the proper position of the diaphragm (some 

 distance under the object — not, as was often thought, close up), as 

 also the inadequacy of this last accessory for use with glasses of 

 higher power than half-inch objectives, Mr. Shephard proceeded 

 to the kernel of his subject — viz., the use of the substage con- 

 denser — and Dallinger was quoted again as saying — " Without a 

 condenser a microscope is either (by construction) not a scientific 

 instrument, or it is an instrument unscientifically used." Defining 

 a substage condenser as " a lens or combination of lenses, either 

 achromatic or chromatic, placed below the stage in order to pro- 

 duce a cone of light," several of the leading types were referred to, 

 and it was asserted that the nearer completeness of correction was 

 attained the better the result. As an e.\ample a much-used chro- 



