PHOTOGRAPHY OF BACTERIA. 161 



high powers ; this is of great importance where there is likely to be 

 vibration from passing traffic. With rapid plates and the highest 

 powers, the exposure has only been two or three seconds, whereas 

 with the parafline lamp it may vary from three to ten minutes, or 

 even longer. 



Walmsley gives the following table for exposures with the 

 par a (fine lamp : 



1| inch objective . . . . 3 to 45 seconds. 

 | - 7 90 



iV . \ 3 minutes. 



i 97 



5- ., & ., < , 



i 4-10 



TO" " " " 



The illuminating apparatus represented in the accompanying 

 engraving (Fig. 78) consists of a lantern which not only moves 

 together with the microscope on the central sliding-board, but 

 can be moved independently to or from the microscope, and be 

 clamped with screws at the requisite distance for obtaining the best 

 illumination. It is provided with two 3-inch condensing lenses of 

 long focus, constructed of optical glass, which is much whiter than 

 that used for ordinary lantern condensers. The lime-cylinders should 

 be of the hardest and best quality, as they give a more actinic light 

 than those made of soft lime. The " Excelsior " lime-cylinders are 

 strongly recommended. They are supplied in hermetically sealed 

 tins which can be easily opened and re-sealed, so that a cylinder can 

 be taken out and used, and the rest preserved for a future occasion. 

 The hydrogen can be obtained by using the coal-gas supplied to the 

 house, and the oxygen should be supplied preferably in a compressed 

 state in iron bottles. Not only are the bottles much less cumbrous 

 than the bags, but a small quantity of gas can be used, and the 

 residue left for an indefinite time ; moreover, the gas is always 

 at hand to be turned on when required. On the other hand, the 

 ivtrntion of unused gas in bags is liable to cause their corrosion, 

 owing, it is believed, to impurities which are carried over in the 

 manufacture of the oxygen. If gas is not laid on in the house, then 

 it also must be procured in a compressed state in bottles. As the 

 blow-over jet is recommended on account of its safety, the bottles 

 should be supplied in this case with a supplementary valve. It is 

 then just as easy and free from danger to employ the compressed 

 ua it is to make use of the house-supply. 



The Camera. A long-focus, half-plate camera is mounted upon 

 a sliding platform. This admits of the camera being pushed up to 



11 



