NATURAL PHILOSOPHY. 149 



cult portion of the operation. An arrangement was used by 

 which the objective could be focussed by the operator, when 

 standing- by the ground-glass plate. 



Col. Woodward was thus able to produce from 12 to 30 nega- 

 tives in an evening's work of 4 hours. About 30 seconds' ex- 

 posure was found necessary for diatoms and Nobert's plate, when 

 the object was magnified 1,000 diameters. In photographing 

 soft tissues, it was necessary to place a plate of ground glass 

 between the condenser and light to prevent interference-phe- 

 nomena, which increased the time of exposure for this class of 

 objects to about 3 minutes. Other powers were found to require 

 proportional times. 



The process employed with the magnesium light is essentially 

 the same as with the electric light. As this light is composed ot' 

 a mixed pencil with rays passing in all directions, there are no in- 

 terference-phenomena caused by it ; but the results with diatoms 

 are inferior to those by the electric light for the same reason. An 

 ordinary two-ribbon magnesium lamp was used, and a magic- 

 lantern condenser served to condense the light in the achromatic 

 condenser of the microscope. The ammonio-sulphate cell was 

 used as with the electric light. An exposure of about 3 minutes 

 was required to produce negatives of tissue preparations with 

 500 diameters. 



Dr. Woodward says further, " In commenting on the above 

 processes it ma} 7 be remarked that for the anatomist and physio- 

 logical investigator the magnesium lamp affords a satisfactory and 

 sufficient source of light for the photography of normal and 

 pathological tissue preparations. The same end can be equally 

 well or even better obtained with the electric lamp, with which 

 also the most difficult test-objects can be satisfactorily reproduced. 

 Where economy of apparatus is the object, the magnesium lamp 

 will be preferred by ordinary workers ; but where much work is 

 to be done, the high price of the magnesium ribbon more than 

 counterbalances the cheapness of the apparatus, and the electri- 

 cal light becomes the most economical. For the information of 

 any practical photographers, who may be employed for work of 

 this character, I may add the following remarks on the chemical 

 process employed in the production of the negatives from which 

 the appended prints were made. An ammonium and potassium 

 portrait collodion, rich in alcohol, was employed, developed with 

 the ordinary solution of iron, and fixed with cyanide of potassium. 

 Where it was necessary to intensify, the hydro-sulphuret of am- 

 monium was resorted to." 



Dr. W. appends to his report 3 prints from negatives of a 

 " Diatom Type Plate, 11 taken, with a Wales' inch and a half, 

 intended to illustrate the relative excellences of sunlight, the 

 light of the magnesium lamp and that of the electric lamp. The 

 first with sunlight is magnilied 40 diameters, the second taken 

 with the magnesium light, 48 diameters, the third by the electric 

 light, GG diameters. " It will be understood at once that, on ac- 

 count of the increase of distance, the second picture would have 

 been slightly less sharp than the first, and the third than the sec- 



