July 1, 1SG5.] 



SCIENCE-GOSSIP. 



153 



A GRADUATING DIAPHRAGM. 



THOSE of our readers who are familiar with 

 microscopic operations are aware how much 

 depends, in the observation of structure, upon the 

 method of illumination. In the investigation of the 

 delicate markings of diatomacese, and the striaj of 

 enamel and miiscular tissue, an immense deal of the 

 worker's success is dependent upon the volume of 

 light which is thrown upon the object. If the flood 

 of rays be very intense, the markings cannot be per- 

 ceived, while if, on the contrary, the illumination be 

 faint, other points of structure are lost sight of. 

 There is, therefore, a necessity for the regulation 

 of the quantity of light reflected to the object. In 

 the instrument at present in use, this end is at- 

 tained by the employment of a piece of apparatus 

 termed "a wheel of diaphragms." This consists 

 simply of a revolving black disk, placed hori- 

 zontally between the stage and the mirror, and 

 pierced by a number of holes (generally 6 or 7) 

 of different sizes. The microscopist, when he 

 desires to alter the illumination, turns the disk 

 round till he tlnds a perforation which suits his pur- 

 pose best. All however will agree that there arc 

 two very serious objections attending the use of the 

 "wheel." In the first instance, though it may help 

 to produce a more suitable form of illumination than 

 that which results from the utilization of all the 

 light reflected fi-om the mirror, its number of aper- 

 tures is hmited, and hence also its application and 

 utilit3^ It not unfrequently occurs that between 

 the two smallest apertures there are several pos- 

 sible ones which would render observation more 

 satisfactory. Secondly — and this is a circumstance 

 of some import, — in changing the aperture, the ob- 

 server loses sight of the object, and has to trust to 

 memory in estimating the advantage he has secured 

 by the alteration. 



These two obstacles have been overcome by Mr. 

 Collins, of Great Titchfield-street, who has devised 

 the diaphragm represented beneath. The instrument 

 consists of a plate of about the size of the stage, 

 and placed below it : this presents an aperture of 

 the same diameter as that of the stage, so that in the 

 illumination of objects under low powers, all the 

 light may be employed. Beneath the first plate are 

 two smaller ones, which move horizontally to the 

 right and left of the stage, and have their proximal 

 borders incised in a wedge-shaped manner. These 

 can be either separated or approximated by means 

 of a lever, which is connected with a screw and 

 milled head, which is situate within easy reach of 

 the observer's finger. When widely separated, these 

 two plates leave a large lozenge-shaped aperture ; 

 but by the application of the screw this may be 

 diminished in the most gradual manner till an aper- 

 ture of scarcely a pin's head's diameter is produced. 



In testing the new diaphragm, a fuU flood of light 

 should first be thrown upon the object (some of the 

 diatomacea; say), and then by the rotation of the 



screw the aperture should be slowly diminished till 

 tlie fine markings come into view. Whether the 

 shape of the aperture may disadvantageously aifect 

 the pencils of hglit as they travel to an object under 

 examination with high powers, is a question which 

 remains to be determined ; but with such objectives 

 as the 5-iuch, and those below it, the illumination- 

 elfects of ColHns's instrument are beautiful. 



Henky Lawson, M.D. 



Peoposed Association oe Amateur MicevO- 

 SCOPISTS. — At a meeting held at 192, Piccadillj^ 

 June 14th, a provisional committee was appointed 

 to draw up a scheme of organization, &c., to be 

 submitted to a general meeting. We are autho- 

 rized to announce that the meeting will take 

 place at St. Martui's National Schools, Adelaide- 

 place, Charing Cross, on Eriday evening, July 7th, 

 at eight o'clock preciselj', to which all persons 

 interested in the establishment of such a society are 

 invited. Tickets may be obtained (free) at 192, 

 Piccadilly ; of Mr. Highley, Green-street, Leicester 

 square ; Mr. Gibson, 9, Lupus-street, Pimlico ; or 

 Mr. Bywater, 5, Hanover-square. 



