FIG. 86.—CAMERA SHOWING ARRANGEMENT FOR OPERA GLASS.
FIG. 87.—VIEW TAKEN WITH OPERA GLASS.
FIG. 88.—VIEW TAKEN FROM SAME SPOT WITH AN ORDINARY VIEW LENS.
As opera glasses are usually constructed for vision only, no attempt is made by the optician to make correction for securing coincidence of foci of the visual and chemical rays of light as in the well-made photographic objective. Hence, it is often found that the actinic focus falls within, or is shorter than, the visual. When this is the case, the proper allowance is easily made after a few trials.
