SCIENCE. 



359 



objective, and subject, to be photographed. It is true 

 we can expose a dry plate for trial, but then we must 

 develop it immediately, and the tirre of developing a 

 dry plate is about three times that of developing a wet 

 one, and a dry plate is also about three times as costly 

 as a wet. one. Therefore the old wet collodion process is 

 the best. 



The collodion to be used should be an old one, and 

 contain some free iodine. I have found that a mixture 

 of "Anthony's red labeled" and " McCollin's delicate 

 half-tone " collodions — both commercial articles — some 

 five or six months old, gives very satisfactory results. 

 The nitrate bath should contain forty grains of niirate 

 of silver to the ounce of water, and should be slightly 

 acidulated with nitric acid. The developer should be a 

 weak one : twelve to fifteen grains of the double salt 

 ammonio-sulphate of iron to the ounce of water, con- 

 taming a few drops of a solution of gelatin and acetic 

 acid as a restrainer. 



After the negative has been fixed in the usual way, 

 with hyposulphite of soda or cyanide of potassium, it is 

 almost always necessary to intensify it, which is easily- 

 done by flowing the plate while wet with a watery solu- 

 tion of iodine until the film becomes white ; then it 

 is to be washed under the tap and flowed with a solution 

 of sulph'de of ammonium, which imparts to the negative 

 a dark brown color, and thus strengthens its printing 

 quality. 



The object to be photographed should be as thin as 

 possible, because the lens will depict only one plane of 

 it, and it should present as much contrast and differenti- 

 ation of its elements as possible ; this is especially the 

 case in animal tissues, and when high powers are used, 

 the focus should be taken with the greatest care for one 

 particular point to be brought out ; a general focus not 

 particularly sharp in any one point, will not give a 

 satisfactory negative. 



The screen upon which the image is focused should 

 be of plate glass, having an extremely fine ground sur- 

 face on one side"— the side next to the object. Such a 

 surface can easily be prepared by flowing the glass plate 

 with a good negative varnish, and when this is set but 

 not yet dry, lightly breathing on it, when an extremely 

 fine and even frosting of the surface will show itself, 

 sufficient to arrest and reflect the rays of light forming 

 the image. 



In photo-micrography, as well as in ordinary micro- 

 scopy, proper illumination of the object is of the greatest 

 importance, and frequently a poor objective will show 

 a better definition in the hands of a skilled manipulator 

 than the best objective can when the light is not 

 properly managed. In this one point lies the difficulty 

 of photo-micrography, and it is the stumbling block over 

 which so many fall who undertake to photograph micro- 

 scopic objects. 



As a general rule the best light is obtained when the 

 back lens of the sub-stage condenser is about half art 

 inch beyond the burning focus of the larger condenser 

 in the shutter, that is about eight and a half inches from 

 this condenser, and when the light is absolutely central. 

 But this distance cannot be strictly adhered to, inasmuch 

 as different objectives require different illumination. In 

 practice, I find that in order to obtain the proper distance 

 of the condenser for a particular objective, it is best to 

 put a blood-slide, upon which the corpuscles are in one 

 layer only, on the stage, and project the image on the 

 screen, moving the condenser backward and forward 

 until, when sharply focused, no concentric rings are seen 

 in the disks. The object to be photographed can then be 

 substituted for the blood-slide, and the light will be 

 found to be all that is desired. (Compendium of Mi- 

 croscopical Technology?! 



PROFESSOR Helmkoltz will issue a collection of his scat- 

 tered scientific memoirs in the autumn. 



PLANTE AND FAURE BATTERIES. 



The annexed illustrations of the secondary batteries, 

 which are exciting so much interest at the present time 

 will, with the accompanying description, enable the 

 reader to understand their construction. At the recent 

 soiree given by the Council and academical staff of 

 King's College, several forms of electric-lightiDg appar- 

 atus were used ; but that which attracted most attention 

 was a battery of forty-four accumulators of Faure's de- 

 sign, working twenty of Swan's limps. The cells were 

 charged in Paris by a Gramme machine, and were ar- 

 ranged in groups of four in cubical boxes, the whole be- 

 ing coupled up in series. The current supplied by this 

 arrangement, shown by a galvanometer in the circuit 

 while the lamps were alight, was about twenty-three 

 webers, and was perfectly steady — the Faure battery 

 yielding an almost equal current during the whole time, 

 until the charge becomes exhausted, when it breaks 

 clown suddenly, without any noticeable warning. Mr. 

 Spottiswoode also uses the Faure battery to woik Swan 



Fic. 1. 



and Maxim lamps in his private house. Figs. I and 2 

 represent the Plants cell. The preparation is as follows : 

 Two sheets of lead (it may be as thin as stcut lead-foil) 

 are laid the one on the other, separated by two strips 

 of india-rubber, the whole being rolled up as shown in 

 Fig. 1. The roll having been completed, the cylinder 

 used in its formation is withdrawn, and it is consolidated 

 by a wrapper of gutta-percha, and inserted in a glass jar 

 filled with water and i-ioth part acid. An electric cur- 

 rent is then made to pass through the cell ; oxygen is 

 given off, and produces a thick cushion of peroxide of 

 lead on one sheet ; hydrogen is given off at the other 

 sheet. If the current with which the cell has been 

 charged be cut off, and the two sheets are connected, a 

 current will be produced, owing to the presence of the 

 oxygen, which leaves the sheet where it has accumulated 

 and attacks and oxidises the other sheet. This secondary 

 current, which is very small at first, gains strength each 

 time the operation is repeated ; in course cf time the 

 surfaces of the sheets are changed, the one being covered 

 with a cushion of peroxide ot lead, the other with lead 

 reduced to a spongy mass. The cell is then complete, 

 and in a state of electrical accumulation. That was 

 Plante's first successful battery. Subsequently he tried 

 the plan of separating the two sheets of lead by canvas, 

 the cell taking the foim of Fig. 2. He then lound that it 

 was necessary to leave a small space between the sheets 

 to provide for the escape of the gases which were pro- 

 duced at the end of the charge ; subsequently india-rub- 

 ber bands were employed in preference to canvas. M. 

 Plante also tried carbonate of lead, minium, &c, but with- 

 out improving upon the results already obtained. The 



