DEFECTS IN NEGATIVES. 145 



beginners as a rule fall into the error of supposing 

 that the field surrounding the object must be opaque. 

 This has evidently arisen from the false impression 

 conveyed by the appearance of microscopic objects in 

 engravings. All photo-micrographs should be trans- 

 parent in the high lights, because there are parts of 

 certain objects, the relative brightness of which may 

 easily escape notice in the microscope owing to their 

 minuteness. For example: the visibility of the 

 structure of the diatomacese depends on the forma- 

 tion of extremely bright spots or lines on a ground, 

 itself possessing considerable brightness, while the 

 surrounding field has an intensity less than the for- 

 mer and greater than the latter. Now if the field be 

 opaque the finer details of the original structure will 

 be lost, not only by a want of variety in the different 

 shades, but also by a blocking up of the minute 

 spaces with a dense deposit. 



Great density is preferable to weakness. The 

 former may be easily remedied by flooding the plate 

 with a solution containing thirty drops of perchloride 

 of iron, twenty grains of bromide of potassium to ten 

 ounces of water, and when the required transparency 

 is obtained, washing the negative rapidly in flowing 

 water. 



A weak image is produced by so many causes, 

 that a complete enumeration would be impossible 

 without entering farther into the subject than is 

 required for our purpose. 



The principal are however : 



