DEFECTS IN NEGATIVES. 138 



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 diatomaceae 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 weak solution of perchloride of iron, 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 ia 

 required for our purpose. 



The principal are however : 



I. A weak bath solution, or too strong a deve- 

 loper with wet plates. 



II. Over exposure, or unsuitable condition of de- 

 veloper with dry plates. 



III. High refractive index of the medium in which 

 the object is mounted. This prevents proper diffe- 

 rentiation of structure both visually and chemically. 

 Even when it does not do so visually it seems to 



