354 Royiil Society. 



exactness for every practical purpose. He then enters into a detailed 

 description of the several parts of an instrument in his possession, 

 constructed on the principles he recommends, referring to the draw- 

 ings which accompany the paper. The magnifying povver may be 

 varied at pleasure, either by drawing out the tubes containing the eye- 

 pieces, or by substituting an eye-glass of different power or differently 

 combined ; and by these changes an uninterrupted range of ampli- 

 fication is obtained from 35 to 800 diameters. No sensible loss as to 

 distinctness is observable, whether the effect is produced by changing 

 the eye-piece or by varying the length of the tubes. The construction 

 of the' instrument admits of the utmost variation of magnifying power, 

 without the risk of losing sight of the object viewed ; and every part 

 which relates to the illumination being wholly detached from the 

 stage, ample opportunity is afforded of rapidly moving the objects, 

 and bringing into view a succession of them, while the light remains 

 the same. Minute directions are given for the employment of the 

 instrument, and its application to various purposes; and great stress 

 is laid on the importance of a skilful management of the light. In 

 stating the results of his experience on this subject, the author takes 

 occasion to advert to some of the sources of fallacy, by which in- 

 cautious observers with the microscope have so often been greatly 

 misled. When a pencil of rays proceeding from an indefinitely small 

 bright portion of an object is brought to a focus by the most perfect 

 object-glass, the image thus formed is in reality not a point, but 

 a small circle, and will always appear as such, if the eye-glass of the 

 microscope be sufficiently powerful. These circles have a considerable 

 analogy to the spacious discs of stars viewed through telescopes. 

 Like the latter, they become much enlarged by diminishing the aper- 

 ture of the object-glass ; and they are also enlarged by increasing 

 the intensity of tilie illumination. The overlapping of contiguous 

 circles of diffusion has given rise to many fallacious appearances, 

 (such as the spottiness which some surfaces assume,) which have 

 been mistaken for globules. This optical illusion has been the basis 

 of some ingenious but visionary speculations on the intimate structure 

 of organic matter. The appearance, in certain directions of the light, 

 of lines on the surface of an object where they do not really exist, 

 may be traced to a similar cause. 



The author proceeds to describe the method he uses for measuring 

 the dimensions of the objects viewed; and notices different test-ob- 

 jects with reference to their affording the means of judging of the 

 powers of the instrument. He next enters into a review of the com- 

 parative merits of various microscopes constructed by Cuthbert and 

 DoUond in this country,and by Chevalier, Selligue, Amici, Utzschnei- 

 der, and Fraunhofer, on the continent. 



The concluding part of the paper is occupied by the development 

 of a principle, from the application of which to the construction of the 

 microscope, the author expects that a still greater extension of its 

 powers will, ere long, be obtained. He remarks, that the circumstance 

 which limits the magnitude of the pencil, admissible with high powers 

 by a single achromatic object-glass, is, that the correction for spheri- 



cal 



