1883] 



MICROSCOPICAL JOURNAL. 



125 



so convenient to use, that the ad- 

 ditional cost becomes a secondary 

 consideration, when amateurs desire 

 to make a few slides to illustrate a 

 subject for an evening's lecture. 



The proper size of plate for use in 

 lanterns is the so-called "quarter 

 size" — measuring 3^ by 4\ inches. 

 We recommend for transparencies 

 Carbutt's " A " plates, because they 

 are not so sensitive as others in the 

 market. 



The developer used is made as fol- 

 lows : I. Dissolve 4 ounces of potas- 

 sic oxalate in 16 ounces of water, 

 acidify with citric acid until blue lit- 

 mus-paper is reddened by the solu- 

 tion, then add 100 grains of citric 

 acid. 



2. Dissolve 2 ounces of ferrous sul- 

 phate in 8 ounces of water, and add 8 

 drops of sulphuric acid. When ready 

 to develop, add one part of the iron 

 solution to four ounces of the oxalate. 



The negative from which transpar- 

 encies are to be made, should be 

 clear, strong, and with good detail. 

 Place the negative in a printing 

 frame. If no printing frame of the 

 proper size is at hand use a large one 

 and put a plate of clear glass in it, 

 upon which the negative mav be laid. 

 Then go into the dark room, put 

 a sensitive plate down upon the 

 negative, and put in the back of 

 the printing frame. For this work a 

 piece of dead-black cloth should be 

 placed over the back of the sensitive 

 plate to obviate reflection, which might 

 seriously impair the result. If the 

 springs of the printing frame will not 

 catch without applying under pressure, 

 hold the back in with the fingers ; but 

 be sure to apply sufficient pressure to 

 insure good contact between the neg- 

 ative and the sensitive film. 



The plates are then held up to the 

 light of a candle or lamp or gas-flame, 

 and the greatest care must be taken 

 not to expose them too long. As a 

 rule, five seconds will be a long expos- 

 ure, at 18 inches from an ordinary gas- 

 flame. 



When the exposure is made, develop 



the plate. The development should 

 be quick, and it must be watched 

 with the greatest care. It is es- 

 sential that the picture should come 

 out leaving the glass around it per- 

 fectly clear. The moment the devel- 

 opment begins to show where the glass 

 should be clear, remove the plate 

 from the pan and wash it. Then 

 proceed with it just as though it was 

 a negative. 



When the negative is quite thin it 

 is a good plan to cover it with white 

 tissue paper before making the ex- 

 posure. This diffuses the light, and 

 often greatly improves the transpar- 

 ency. 



The great tendency of beginners in 

 this kind of work is to make pictures 

 that are too dense. For a lantern 

 very little density is required. If the 

 detail is good that is all that is de- 

 sired. It will all show on the screen, 

 even though it seems too weak on the 

 glass. 



There is also a great tendency to 

 expose the plates too long. Three 

 or four seconds seems a very short 

 time for printing, but it is usually 

 quite sufficient. Mr. George E. Davis 

 has even made a good transparency 

 with an exposure of only twenty sec- 

 onds to the light of a small alcohol 

 lamp. It is true he used a more sen- 

 sitive plate than the ones here recom- 

 mended, but the experiment shows 

 what actinic force there is in a very 

 weak light. 



A few words now about the appli- 

 cations of this process. Those who 

 have used the projecting microscope, 

 are well aware of the difficulties 

 in the way of its general use even 

 in halls of small size. High-pow- 

 ers cannot be used with it with 

 any satisfaction, and instead of being 

 able to select objects to illustrate a 

 lecture, the lecture must be, to some 

 extent, arranged to conform to the 

 capabilities of the lantern. We do 

 not imply that this will always be so. 

 But, taking the facts as they are at 

 present, it is certainly much better to 

 use photographs of microscopic ob 



