NATURAL PHILOSOPHY. 147 



focus. These photographs confirm the opinion of Mr. Wenham 

 and Prof. Rood, as to the cirouhir nature of the markings on P. 

 angulaium. It is remarkable that the markings appear hexagonal 

 in both the small pictures, if viewed with the eye at the visual 

 distance ; while on close inspection, or with a lens, they are seen to 

 be circular in the pictures; with 19.0J0 diameters, the circular 

 shape of the markings is ver}- plain, but, if viewed from a coa- 

 siderable distance or with a concave lens, they appear hexagonal. 



NEW n^LUMINATOKS FOR OPAQUE OBJECTS. 



H. L. Smith, of Kenyon College, contributes a paper on this 

 subject to " Sillimans' Journal," for September, 1865, from which 

 the following are extracts : — 



" In attempting to study the structure of the diatomaceous frus- 

 tule, I found it impossible to view it with high powers as an opaque 

 object by any means hithei'to devised. In a valuable paper on 

 the scales of the podura (' Mic. Joui'.,' N. S., vol. 2, p. 8G), Mr. 

 Richard Beck has stated that there is no diiBculty in viewing them 

 as an opaque object, with the one-eighth-inch objective and con- 

 densers righth* placed. Any illumination of diatoms thus ob- 

 tained is almost useless, from the great obliquity of the light, 

 and, with powers higher than the one-eighth-inch, is quite impos- 

 sible. Mr. Ross's ingenious arrangement, suggested by Mr. 

 Brooke, of a plain retlector, flush with the front surface of the 

 objective and receiving light from a truncated ellipsoidal reflector 

 below, is so exceedingly diflicult to use, and only with a special 

 mounting of the object, that it has never been generally adopted. 

 Mr. "NVenham's method is entirely inapplicable to diaroms, inas- 

 much as it depends upon the total reflection of the light from the 

 under surface of tlie glass cover of a mounted object, and in such 

 case the diatoms, from their transparency, and the near coinci- 

 dence of refractive index of silex with that of the mounting fluid, 

 throw back but a feeble light, and are nearl}- invisible. The use of 

 the well-known collimating eye-piece suggested to me the idea of 

 making the objective its own condenser; and upon communicating 

 this idea to ilr. Wales, already well known for the excellence of 

 his objectives, he at once sent me a trial instrument. This first 

 illuminator proved so far successful that I was induced to perse- 

 vere; and, with his assistance, an 'illuminator' has been con- 

 structed which gives entire satisfaction, and answers admirably 

 with all objectives from four-tenths to one-fiftieth. 



" It must be borne in mind that there are certain difficulties to be 

 overcome in this mode of illumination, the chief of which is the 

 reflection of the light from the posterior surface of the back com- 

 bination of the objective. All the difiiculties are now surmounted, 

 and there is no trouble in viewing diatoms, or other objects, 

 mounted dr}-, and uncovered, with tiie highest powers of the mi- 

 crosco2Je, and with- abundant illkmiination ; and this without any 

 trouble in mounting the object on little disks or pins, but using 

 the ordinary three-inch by oue-ineh slide. 



