. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . Fig. 119.—Diagrams of the Photoptometer of Foerster. into the box comes through a window F, the aperture of which can bechanged. Inside the compartment, screened off by the window F, isa standard candle L. The minimum aperture of the window F, whichof course regulates the amount of illumination falling upon T, per-mitting the observer to see the black marks gives the threshold value.The results are subject to the variations which arise from variousconditions of retinal adaptation. 230 PHYSIOLOGI

This image is a public domain image, which means either that copyright has expired in the image or the copyright holder has waived their copyright. Alamy charges you a fee for access to the high resolution copy of the image.

This image could have imperfections as it’s either historical or reportage.

. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . Fig. 119.—Diagrams of the Photoptometer of Foerster. into the box comes through a window F, the aperture of which can bechanged. Inside the compartment, screened off by the window F, isa standard candle L. The minimum aperture of the window F, whichof course regulates the amount of illumination falling upon T, per-mitting the observer to see the black marks gives the threshold value.The results are subject to the variations which arise from variousconditions of retinal adaptation. 230 PHYSIOLOGICAL OPTICS 189. Aubert determined the threshold of the normal eye; he foundthat the weakest light that can be distinguished is that of a sheet ofwhite paper illuminated by a candle placed at a distance of from200 to 250 meters. But the luminous sense varies within wide limitswith the state of the retinal adaptation; it is indispensable, therefore, that the state of the adaptation be known in determining the lightsense. In fact a determination of the threshold values made aftervarious periods o