(See Amidol.)


A small piece of diamond mounted in a handle used for cutting glass.


Lumiere's preparation of diamidophenol, or amidol.


Originally a plate having a center of a certain diameter. A series being arranged with openings of relative proportion to the focal-length of the lens to which they belong.

Iris Diaphragm

An adjustable stop with overlapping leaves which open and close from the center. Manipulated by revolving a ring or lever outside the lens, which gives any desired opening or aperture.

Diaphragm Shutter

(See Shutter, Diaphragm.)

Star Diaphragm

A metallic star suspended so as to partially obstruct the rays of light from striking the center of the lens. The object is to secure equality of illumination when using extremely wide-angle lenses.

Waterhouse Diaphragm

A stop cut from a thin sheet of metal and inserted in a slot in the lens mount commonly known as Waterhouse stops.


A positive made from a negative.

Dichroic Fog

(See Fog, Dichroic.)


All DIFFICULTIES, their cause, prevention and remedy, are given immediately following each lesson, or subject, and will be found indexed under such subject headings in the various volumes.

Diffusion of Focus

(See Focus, Diffusion of.)

Diffusion of Gases

The passing of one gas into the space occupied by another. Diffusing Screen

(See Screen, Diffusing.)

Diffusion of Light

VI. (See Light, Diffusion of.)

Diffusion of Liquids

(See Liquids, Diffusion of.)


To make thin with water.


A newly introduced developing agent rendering negatives of black color and fine gradation.



White scales. Soluble in hot alcohol and ether; practically insoluble in water, but soluble when alkalies or sulphite of soda has been added to the water. A convenient form of developer.


Breaking up of white light into its component colors. (See Prismatic Colors.)


The part of the view which appears farthest from the eye. (See Perspective, Aerial.)

Line of Distance

A straight line drawn from the eye to the principal point in the plane of the picture.

Middle Distance

A point in a picture midway between the nearest and most remote objects.

Point of Distance

The point of the picture where the visual rays meet.


A process whereby a liquid, or even a solid, is heated in order that certain constituents of it may be given off in vapor, and then by a suitable apparatus condensed as a liquid.

Destructive Distillation

The heating of non-explosive or non-volatile organic bodies in a retort.