WITH THE MICROSCOPE. 25 



spherical condenser for examining objects marked with very fine 

 lines by oblique light. Trans. Mic. Soc. 1861, p. 59. The same 

 observer has recently modified his instrument by the addition of 

 another lens, by which arrangement he is able to obtain a ray of 

 light of greater obliquity than is possible by ordinary methods of 

 proceeding. Micr. Journal, January, 1867, p. 3. 



SO. New Webster Condenser. Lately a form of achromatic con- 

 denser, which passes by the name of ' Webster's,' like the eye-piece 

 used for a condenser, lets a flood of light upon the object, has been 

 much improved by Mr. Highley, Mr. Collins, and other makers. 

 Mr. Collins' ingenious arrangement for altering the size of the aper- 

 ture of the diaphragm, instead of using the plate with holes in it, 

 will be understood by reference to fig. 59, pi. XIII. It seems to me 

 likely that this will supersede other plans entirely. This condenser 

 is well adapted for working with the binocular. Mr. Collins is 

 endeavouring to increase the angular aperture by the addition of a 

 third lens, and render it really achromatic like Reiner's eye-piece 

 above referred to. 



Although it seemed to me desirable to refer to the above different 

 methods of modifying the illumination of objects, it must not be 

 supposed that the delicate instruments which have been described 

 are essential for beginners or for ordinary observation. The student 

 may even pursue some branches of original investigation in which 

 high powers are not required, without employing one of them. In 

 special enquiries, however, great advantage has resulted from the use 

 of some of these instruments, and no one would attempt to undertake 

 certain researches, as for instance, upon the nature of markings on 

 diatoms or other delicate structures, without making himself familiar 

 with the different effects resulting from their use, and he would 

 probably soon find, that by modifying the plan which gave the most 

 favourable results still better definition was to be obtained, or new 

 facts were to be demonstrated. 



4O. Examination of Objects.- It is instructive for the observer 

 to subject specimens as for example, granules of fine sand or pow- 

 dered gypsum, potato starch, or arrowroot, to examination in four 

 different ways. i. The surface of the object may be examined by 

 reflected light brought to a focus upon it by means of a bull's-eye con- 

 denser. 2. The light may be reflected upon it from a Lieberkuhn. 

 3. The light may be transmitted through the object after it has been 

 reflected from the surface of the mirror. And, 4. The object may be 

 placed under the influence of polarised light, with and without a 

 selenite plate. 



The conclusion arrived at with reference to the nature of the 



