208 The Microscope. 



While neither method can be criticised, it is apparent that the wet 

 plate brings out the tissue-elements a little more clearly and 

 sharply defined than the dry plates, and, although in its application 

 the wet process is not quite so easy as the other, we are inclined, 

 judging from Dr. Gray's photographs, to favor that. From Dr. A. 

 G. Field, Des Moines, la., blue prints and photographs of urinary 

 crystals, etc. 



TECHNOLOGY. 



METHOD OF INTENSIFYING THE RESOLVING POWER 

 OF MICROSCOPE OBJECTIVES. 



"IX/TR. G. D. hirst describes a simple way of vastly improving 

 ■^ -*- the definition of objectives on close-lined test objects 

 which has lately come under his notice. The credit of the discovery 

 is due to Mr. Francis, of Sydney. 



Take a valve of, say, Amphipleura pellucida, and having got the 

 best results obtainable with mirror and condenser, let the analysing 

 prisnj belonging to the polarizing apparatus be placed over the eye- 

 piece, and rotated until it darkens the field, which it will do, though 

 not to the same extent as when vised with the polarizing prism. On 

 carefully focusing the diatom, the lines will show themselves with an 

 extraordinary increase of definition. Valves that, without the aid of 

 the prism, only show a washy sort of resolution, will now show the 

 lines as black as the bars of a gridiron. 



On P. angulatum by central light, the result is also splendid. 

 The same effect can also be obtained, though perhaps to a slightly 

 inferior degree, with the objective, or, as it is placed in some stands, 

 in a sliding box in the body of the microscope; in the latter case, as 

 it cannot be rotated, the valve of A. pellucida should lie horizon- 

 tally. For general purposes, it is better for the prism to fit over the 

 eye-piece, as besides giving better definition in that position, with a 

 diatom like P. angulatum and prism over the objective, the diffrac- 

 tion spectra would be cut out of the top and bottom of the back lens 

 and the effect spoiled. Of course, in the case of A. pellucida, vsdth 

 the valve lying horizontally, it does not matter, as the dioptric ray 

 and single spectrum are not cut off in any way by the prism or the 

 box in which it is set. The prism has the effect of greatly diminish- 

 ing the light of the dioptric beam; at the same time it scarcely 

 touches that transmitted by the diffraction spectra. 



