rectilineal action of light. On account of the latter circumstance the impression itself leads directly to the place of the cause, and since the eye is capable of perceiving with the greatest exactness and at a glance all the fine distinctions of light and shade, colour and outline, and also the data in accordance with which the understanding estimates distance, it thus happens that in the case of impressions of this sense the operation of the understanding takes place with such rapidity and certainty that we are just as little conscious of it as of spelling when we read. Hence arises the delusion that the sensation itself presents us directly with the objects. Yet it is just in sight that the operation of the _understanding_, consisting in the knowledge of the cause from the effect, is most significant. By means of it what is felt doubly, with two eyes, is perceived as single; by means of it the impression which strikes the retina upside down, in consequence of the crossing of the rays in the pupils, is put right by following back the cause of this in the same direction, or as we express ourselves, we see things upright although their image in the eye is reversed; and finally by means of the operation of the understanding magnitude and distance are estimated by us in direct perception from five different data, which are very clearly and beautifully described by Dr. Thomas Reid. I expounded all this, and also the proofs which irrefutably establish the _intellectual nature of perception_, as long ago as 1816, in my essay “On Sight and Colour” (second edition, 1854; third edition, 1870), and with important additions fifteen years later in the revised Latin version of it which is given under the title, “_Theoria Colorum Physiologica Eademque Primaria_,” in the third volume of the “_Scriptores Ophthalmologici Minores_,” published by Justus Radius in 1830; yet most fully and thoroughly in the second (and third) edition of my essay “On the Principle of Sufficient Reason,” § 21. Therefore on this important subject I refer to these works, so as not to extend unduly the present exposition.