Practical Psychology

FOR STUDENTS OF EDUCATION

Fox, Charles

• Editore: Routledge
• Anno: 2013
• ISBN:
  9781136321511
• Edizione: 1
• Argomento: Medicina
• Categoria: Prestazione di assistenza sanitaria

This is Volume VII of thirty-eight in the General Psychology series. First published in 1928, the subject matter offers practical educational psychology for students, with experiments, recorded data, statistical manipulation and discussion.

PRACTICAL PSYCHOLOGY

PART I
EXPERIMENTAL

CHAPTER I
SENSATION AND PERCEPTION

Visual Acuity

Experiment 1

The standard of visual acuity is defined as the power to distinguish black objects on a white surface which subtend an angle of 5 minutes on the retina. A line of 1.45 mms. at a distance of 1 metre from the eye subtends this angle; and such a line can be distinguished at the distance named by the so-called ‘normal’ eye. When letters are used, as in Snellen's type sheets, the size of the letters is such as to produce an angle of 5′ vertically and horizontally, the strokes and spaces between the letters subtending an angle of 1′. In the diagram on the next page the lines are 2.9 mms. and therefore subtend an angle of 5′ at a distance of 2 metres, whilst the width of the lines subtends an angle of 1′.

The diagram is placed vertically in front of the subject, who stands with his back to a large northern window with a clear skyline, at a distance of about 4½ metres from the lines. One eye is shaded but not closed, and by means of a slit in a cardboard screen each of the five horizontal series of lines is exposed by the experimenter in a prearranged order. The subject calls out the number of lines, and if he makes any mistake he moves nearer about ½ metre each time until he reaches a critical point at which he makes no mistakes. At each distance, in order to prevent guessing, two blanks are introduced at irregular intervals into the series, and these must be correctly discriminated. The lines are shown in a different order at each distance. When the 5 series and

FIG. I.

the blanks have been correctly discerned the subject moves nearer to the diagram and the whole process is repeated as he steps backward ½ metre at a time. In this way another critical point is found. The distances are measured by a cord from the diagram to the eye. The mean of the two critical distances is the subject's threshold. The standard distance for these lines is 2 metres, and

Each eye should be tested in turn, with and without glasses. The eye is easily fatiguable, and therefore subject and experimenter should change places after each eye is tested.

A useful topic for investigation may be made out of the following extension of the experiment. The lines are printed in different colours and the threshold for different colours may be determined. A table is drawn up:

Colour	Threshold (Cms.)	Percentage of Black
Black Blue Red Green Yellow

The figures in the third column are found by dividing each threshold by the value of the threshold for black, in good daylight. The usual order is that given in the table; but there are interesting individual variations worthy of careful investigation.

Note on the Limiting Method.—The method used in this experiment is a crude instance of the limiting method, or the method of minimal change. As the subject approaches or recedes from the diagram, the size of the retinal image, which may be called the stimulus, increases or decreases. It is obviously the same whether the diagram is moved or the subject moves. The magnitude of the stimulus which is just discernible, or what comes to the same thing, the distance at which the lines may be separately distinguished, is called the liminal (limen = threshold) stimulus. The sensation, as it were, is lifted over the threshold from sub-consciousness to consciousness. The stimulus must attain a certain magnitude in order to produce a definite sensation. The limiting method is one of the psycho-physical methods, the object of the procedure being such as to ensure that the threshold is determined fairly and, as far as possible, to eliminate random guesses. The stimulus, therefore, is slowly increased or decreased in size. There is an indefinite region of uncertainty between the critical points where the subject sometimes succeeds and sometimes fails, and by the limiting method we attempt to determine as exactly as possible the point within this region where the threshold is just crossed.

Auditory Acuity

Experiment 2

The object of this experiment is to determine the threshold for auditory acuity.

The method used, as in the case of vision, is a simple form of the limiting method. The experimenter prepares four series of 10 figures each and practises repeating them in a ‘forced whisper,’ saying each number at the end of an expiration. When he can do this readily, he seats the subject at a distance of 20 feet away in a large room with his right ear towards himself. The left ear should be plugged with a rubber stopper, and the mouth of the subject must be closed. A few test figures are now repeated in forced whisper to accustom the subject to the conditions. Before whispering each figure the experimenter makes a signal by a sharp rap so as to engage the attention of the subject. The subject is instructed to write down each number as he hears it, and to make a stroke if he fails. In this way a series of 10 numbers is repeated, then a short rest is taken, and the next 10 is repeated, and so on. When the whole four series is finished the experimenter moves 5 feet towards the subject and repeats the whole four series again. The operation is repeated, at successive distances, until the experimenter is 5 feet from the subject. He then steps backwards 5 feet at a time and repeats the series in the same way.

Both ears are tested in this way; and subject and experimenter change rôles after each ear is tested, for the sake of variety.

Binaural hearing may be measured in the same way, by allowing the subject to face the experimenter, the eyes being closed or shaded.

By sorting out the number of figures correctly heard at each distance, a rough idea of the threshold of auditory acuity may be obtained. That distance at which 80 to 90 per cent. of the figures are correctly recorded may be arbitrarily selected as the threshold. As in the previous experiment, the threshold is determined both inwards and outwards and the mean of the two is taken as the final result.

A similar method may be employed with a stop watch. In this case the experimenter moves forwards or backwards ½ to 1 metre at a time. At each distance the watch is set going, after a prearranged signal, 5 times; but 2 ‘catches’ are introduced at each distance. The subject calls out if he hears the watch. The threshold is the distance at which he makes no blunders. The watch test produces much more variable results and is greatly affected by practice. If time permits the whisper test should be employed in preference to the watch test.

Perception

Experiment 3

The purpose of this experiment is twofold: to find out what difference is made in observation by a preliminary systematic knowledge of what one is about to observe, and also the part played in observation by an adequate terminology.

The material selected consists of suits of armour (see Appendix I.), since these have a definite describable form and each part has a technical name. In these respects the material is comparable in use to the description of an animal in zoology or a plant in botany. The experiment is suitable for class work and may be applied...