172 



HA RD WI CKE 'S S CIE JVC E- GO SSI P. 



carbon mixed with carbon-dioxide. These are held 

 together top and bottom by two india-rubber rings 

 which prevent contact with the hollow cylinder of 

 zinc by which they are surrounded. There is no 

 porous cell. The electrolyte used is merely sal- 

 ammoniac dissolved in water. There is nothing acid, 

 poisonous, corrosive, or objectionable, about it. 

 Ammonia is given off, but in such small quantities 

 that I have never noticed any smell. Holders for the 

 lamp are sold at 21s. each, including stand; but I 

 have contrived a holder to fix on to the stand of my 

 bull's eye condenser for 8s. The condenser has a ball 

 joint top and bottom. Into a small hole, which 

 happened to be ready made in my Crouch's con- 

 denser, just above the upper ball joint and just below 

 the lens, a small steel rod 2 in. long fits with a 

 shoulder, and a screw thread with a minute nut. It 

 can turn round in the hole. At the end of the red is 

 a cradle-joint (a universal ball joint would be better, 



(cost 5</.) of black silk covered ladies' iron bonnet 

 wire as fine as possible. I have divided this into 

 six pieces, and wound each into a double coil, that 

 is, the wire should be doubled in the middle and 

 the length of double wire thus obtained wound in a 

 spiral. There is no self-induction in a coil so wound. 

 The six coils might be arranged in a circle between 

 two circular boards, and the ends after unwinding 

 the silk inserted through holes in the upper board 

 so that the end of one touched the end of another. 



Then, if the brass handle of a dining-room bell-pull, 

 cost is. T,d., be fixed so as to turn in a circle on] a 

 spindle in the centre of the circular boards, contact 

 may be made or broken with one, two, three, four, 

 five, or six coils, by turning the handle. One of the 

 battery wires is connected to the beginning of the 

 first coil, and a wire from the lamp is connected with 

 the bell-pull near the centre hole. I have made 

 this, and find that it does answer. See Jenkin's 



-9£- ' 



<■ II?. r * 



Fig. 103.— Micrographic Camera, microscope used as slide holder and electric battery, c, camera ; T, tin and indiarubber cone ; 

 B._ book to raise camera; E, electric light; w w, double covered wire; J, jars of battery; z, zincs; o, objective. Scale, 

 1 inch to i foot. 



Lut the cradle joint answers every purpose). At- 

 tached to the joint is a short tube of brass, as made 

 for stage-forceps. Through the tube slides a steel 

 rod 5f in. long over all. At one end of this steel 

 rod I have attached the forcep-jaws of the ordinary 

 stage-forceps, at the other a cylindrical piece of ebonite 

 about I in. long, fixed at right angles to the rod. To 

 the sides of the ebonite two oval-oblong slips of brass 

 are screwed with two brass screws each. The screws 

 must'not touch the steel rod, and the distance between 

 the brass slips is jj in., the diameter of the lamp socket. 

 Under one screw at each side is inserted one strand of 

 a two-foot length of green silk covered flexible double 

 copper wire. The wires to the battery need not be 

 so ornamental. Mr. Stearn recommends a resistance- 

 coil to be placed in circuit by which the intensity of 

 the light may be controlled. As this coil is not 

 absolutely essential and rather costly, I have made 

 one myself, of two coils of twenty-four feet each 



" Electricity and Magnetism," p. 234 (Longmans, 

 3f. 6(f.) The only precaution to be observed is to 

 connect one cell at a time, so as not to break the 

 carbon by the full strength of the current. 



I am perfectly satisfied with the electric light, it is 

 quite steady, very convenient, can be used close to 

 the object, and shows colours like daylight. I believe 

 that it is perfectly adapted for photography. The 

 Rev. W. H. Dallinger, F.R.S., tells me better light 

 for the microscope can be obtained in other ways, 

 but I like it more than anything I have seen. 



Tqpton Elms, Sheffield. Bernard Hobson. 



Species of Chrysomela. — Your correspondent, 

 T. D. A. Cockerell, asks for information about a 

 Chrysomela like polita. His description exactly 

 suits C. polita. Has he not hitherto mistaken 

 C. staphylaca for C.polital—W. C. Hey. 



