PHOTOGRAPHIC INTRODUCTION 7 



times to be positive whether the negative has been 

 over or under-exposed, as the appearances pre- 

 sented do not conform as much as would be 

 expected with those presented by over and under 

 exposure in negatives of other subjects. Speaking 

 again quite generally, the best guide we have 

 found is to examine the background of the de- 

 veloped plate about whose exposure there is doubt, 

 taking no notice of the appearance of the bacilli. 

 If the background of the negative is of the 

 right density for producing a grey ground in the 

 print a matter which experience will soon teach- 

 then, if the proper contrast glass has been used, the 

 bacteria should appear well arid clearly defined and 

 the print should be " plucky " and sharp, presuming, 

 of course, the focussing has been correct. But if the 

 background be faint in the negative the final pic- 

 ture will never be a good one, but usually flat and 

 feeble in contrast. Here it is necessary to point out 

 that the judgment of the photographer is not unfre- 

 quently put to the test as to the amount of contrast 

 really required to make the bacilli stand out in the 

 print. In some cases an almost white background is 

 best, whilst at others a grey one is to be preferred. 

 We have in cases been obliged to try the direct 

 experiment, and choose that which appeared to 



