-L90 SUMMARY OF CURRENT RESEARCHES RELATING TO 



developer gives a deposit of a bluer colour which allows the light to pass 

 much more readily. 



Then arises the important question of exposure. The action of the 

 developer reduces to a metallic state the sensitive salts of silver present 

 in the film which have been rendered reducible 'by the action of light: 

 hence, in order that the developer may do its work efficiently, and 

 reduce the requisite amount of silver, the exposure must be generous 

 enough to allow full scope to the developer. If too little is given, no 

 amount of prolonged development can make up the shortage, for, in 

 other words, density and detail are settled by the exposure, and 

 development simply makes visible the action of light. 



With regard to any difficulty in securing density with high mag- 

 nifications, it must be remembered that the plate used is not concerned 

 with size of image, but with light-action only, so that if owing to high 

 magnification, the light reaching the plate is dim, sufficient exposure 

 must be given to compensate for the slow action of what light is passing. 

 Even if exposed to so poor and slow a light as the red lamp of the dark- 

 room for a sufficient length of time, full density (light-fog) will be 

 obtained on any plate by development. 



Pybo-Soda Developer. 



(A) Pyrogallic acid — crystals 1 oz. 



Potass. Metabisulphite 1 dram 



Potass, bromide 1 ,, 



Water to 80 oz. 



(B) Sodium carbonate — crystals 8 oz. 



Sodium sulphite 8 ,, 



Water to 80 „ 



To develop : take equal parts of each. Used at. a temperature of 

 70° F. full density will be obtained in five minutes if the exposure has 

 been correct. 



Strong prints may sometimes be secured from thin negatives by 

 making them on ' ; gaslight " paper, though at the expense of " blocked 

 shadows " ; but it is far preferable to aim at the perfect negative, for 

 then all is plain sailing. For photomicrographic prints " Enainrno " 

 paper is all that can be desired. 



( 6) Miscellaneous. 



Possible Methods of Ruling used by Nobert.* — Nobert's ruled 

 test-lines at one time, says John M. Blake, were the subject of much 

 interest and discussion. Thev were regarded as marvels of mechanical 

 skill. The rulings designed and used as a test of the performance of 

 microscopic objectives were looked upon as the most wonderful of his 

 productions. 



It has been stated that, after his death, an investigation of his 

 apparatus did not reveal his method of work. Possibly the apparatus 



* Amer. Journ. Sci., xxxviii. (1914) pp. 147-8. 



