208 MEMORANDA. 



is blackened so as to prevent the passage of all direct 

 light to the object. The arrangement is the same as that 

 employed by M. Amici for the illumination of objects with 

 one of the colours of the spectrum, by making use of the 

 solar light, and making the cone more pointed, and of flint- 

 glass, in order to disperse the light sufficiently. In the cone 

 here represented, which is made of crown-glass, the dispersion 

 is not sufficiently great to give coloured rays, and the images 

 consequently are perfectly colourless. — Nachet. 



On ail Oblique-Light Illuminator. — In cases where a very 

 oblique light is required in order to see very delicate lines, 

 the mirror and prism of Amici are often insufficient ; or at any 

 rate, they demand long and tedious adjustment, even with 

 objectives which bring out with tolerable facility the most 

 difficult tests. I have found that a very simple means of 

 effecting this object consists in a slight modification of the 

 old illuminator of Mr. Kingsley. The upper lens should be 

 covered with a thin plate, having a perforation at the margin 

 of the lens, so as to allow only a very sharp pencil to strike 

 the object. I would remark that a notable difference exists 

 between the diaphragm placed below the eclairage, as is gene- 

 rally done, and the arrangement now proposed, which has 

 the advantage of cutting the luminous pencil after it has 

 undergone all its evolutions in crossing the lenses. This 

 combination has the focus sufficiently long to allow of its 

 employment on preparations mounted on glass of the ordi- 

 nary thickness. - 



With the objectives Nos. 7 and 8 (tVth. and -j^th inch), the 

 lines on Grammatophora subtilissima, the undulating lines of 

 Surirella gemma, and those of Navicula affinis (Amici' s test), 

 are immediately resolved with the aid of this arrangement of 

 the illuminator. It moreover possesses the advantage of 

 being readily applicable to instruments in which the. stage is 

 so thick that it is impossible to obtain an illumination suffi- 

 ciently oblique for delicate researches. — Nachet. 



Amphipleura pellucida. — In examining a series of fourteen 

 slides of this diatom with an eighth objective, without any 

 accessary apparatus, I have been enabled to come to a satis- 

 factory conclusion that it is a sad misrepresentation to set 

 down the lines so high in the scale as 130 in -001". 



That the lines of some are exceedingly fine, and beyond 

 my present means of giving a numerical limit, yet a few 

 shells may bp counted at 42, and many at (50. 70, and 80, in 



