11 



from circles of confusion and chromatic dispersion, such as a 

 fine pencil of rays admitted through an exceedingly fine 

 aperture from the direct solar beam. 



(h) By analysing the aberration of the annular surfaces 

 of the objective, and selecting such areas and annuli as are 

 the more perfect in their operation. 



(i) Lastly, by abstention from pressing the powers of the 

 objective beyond distinct vision, a canon universally adopted 

 by astronomers, but too much neglected by microscopists. 

 In telescopes every inch of diameter is generally considered 

 to barely admit a power of 100, a 10-inch objective scarcely 

 allowing a poAver of 1000. In microscopes the power may 

 be similarly estimated for useful effects by taking 100 times 

 the reciprocal focal length, one eighth giving 800 diameters. 

 There are two other points to which I beg to direct the 

 attention of microscopists, as worthy of their best efforts and 

 scientific research. 



(k) The spherical aberration, both lateral and longitudinal ; 

 which will be improved — 



(l) By extending the visual focal distance of deep ob- 

 jectives, and so withdrawing the face-glass from its extreme 

 and dangerous proximity to the " covering glass." 



(m) By greatly increasing the depth of focal vision, and 

 calculating its amount. With a 3-inch objective it is 

 possible to view, at one and the same instance, both surfaces 

 of a thin covering glass ; and the marvellous and delightful 

 perspective view into deeper parts of insects given by Koss's 

 4-inch objective illustrates the same principle of the ad- 

 vantages of increasing focal depth. These points having 

 occupied my attention for many years, I hope shortly to 

 communicate the methods adopted to accomplish these 

 desiderata. 



There is a great deal of interest in the subject of definition, 

 because it is one common to the sister sciences of the astro- 

 nomer and the microscopist. To the former " dividing power," 

 " definition/' giving fixed stars a round disc, resembling a 

 bright spangle placed upon black velvet, and '* diffraction 

 rings," and lastly " nebulosity" and haze, have all their spe- 

 cial, though humbler, representatives in the microscopic field. 

 And in order to obtain precise ideas, it is wise to proceed 

 from the known to the unknown ; I therefore beg to suggest 

 the study of these points in the images formed of given' and 

 known objects by minute lenses and high microscopic power. 

 For the information of those interested in this point I have 

 calculated the diameters of the circles of least aberration for 

 parallel rays for minute lenses of the following dimensions. 



