Recent Experiments in Psychology

By Leland W. Crafts; Theodore C. Schneirla et al. | Go to book overview

CHAPTER VIII
THE DUPLICITY THEORY OF VISION

INTRODUCTION
The retina of the human eye contains two kinds of receptor cells, the rods and the cones. These two types of cells differ with respect to their structure, their function, and their distribution within the retina. Within the fovea centralis1 there are many thousands of cones, but no rods. As one passes from the center toward the periphery of the retina, the proportion of rods to cones steadily increases until, at the extreme periphery, rods are thickly distributed and there are few, if any, cones.2According to the "duplicity theory" of von Kries, the cones and rods of the retina have two important differences in function: First, the cones are the receptor cells which function at daytime levels of light intensity, whereas the rods are the receptors for "twilight vision," i.e., for vision at the low intensities of illumination which characterize twilight and night conditions. Second, the cones are the cells which are so constituted that they enable us to perceive colors and to discriminate color differences, whereas the rods make possible only colorless, or achromatic, vision. The evidence supporting this duplicity theory may be summarized as follows.
1. Foveal Night Blindness. It has been shown that under deep twilight conditions of illumination the fovea is in effect a scotoma, i.e., a blind area. Many years ago astronomers observed that they could not see very dim stars when they were looking directly at them but that these same stars became visible when they turned their eyes away to view some neighboring star. Since they were now fixating the neighboring star, the light from the dim star would fall outside
____________________
1
The fovea centralis is the small, central "area of clearest vision" upon which is impressed the retinal image of small objects or figures which are directly "fixated" by the observer.
2
The total number of rods exceeds that of the cones by many millions. Estimates of the number of rods vary from 60,000,000 to 130,000,000, where the estimated num­ ber of cones is only about 7,000,000.

-122-

Notes for this page

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

Items saved from this book

This book has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this book

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Notes
Cite this page

Cited page

Style
Citations are available only to our active members.
Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited page

Bookmark this page
Recent Experiments in Psychology
Table of contents

Table of contents

Settings

Settings

Typeface
Text size Smaller Larger Reset View mode
Search within

Search within this book

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Help
Full screen
/ 512

matching results for page

    Questia reader help

    How to highlight and cite specific passages

    1. Click or tap the first word you want to select.
    2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
    3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

    OK, got it!

    Cited passage

    Style
    Citations are available only to our active members.
    Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

    1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

    Cited passage

    Thanks for trying Questia!

    Please continue trying out our research tools, but please note, full functionality is available only to our active members.

    Your work will be lost once you leave this Web page.

    Buy instant access to save your work.

    Already a member? Log in now.

    Oops!

    An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.