^10 TEAXSACTIO^'S OF THE A3IERICAN INSTITUTE. 



site side of the card, and then suspending it from the top by a 

 thread, so that it will hang in a vertical line. After twisting the 

 thread sufficiently to cause a rapid rotation of the card, it is left 

 free to move, and when the two halves are each presented to tho 

 eye eight times per second, their images become comlnned on the 

 retina, tmd form a complete picture. Various plans for producing 

 the same effect will doubtless be sufjo-ested to the reader. i 



There is another class of visual impressions which are reproduced 

 involuntarily when the eye is overtasked. The retina is generally 

 so delicately constructed as to be sensitive to many hundred different 

 tints, and it responds to the undulations causing them, so as to con- 

 vey to the mind a distinct impression of each. When, however, 

 the retina is exposed to the direct rays of a strong light — the sun, 

 for instance- — it is thrown into such violent internal agitation that a 

 brilliant image of the object from which the light emanated will 

 be visible for several minutes when the eyelids are closed, and will 

 sometimes reappear in diversified colors, which are generally com- 

 plimentary to those immediately preceding. A remarkable pecu- 

 liarity in these phenomena is the reappearance of the latent image, 

 which may even be recalled or reproduced by an act of the will, 

 after the eye, has received many other impressions. It is not 

 improbable that the spectral apparitions, which were formerly said 

 to haunt humanity in "the witching time of night," were the 

 offspring of a disordered and over-excited retina; thus ever}-^ impal- 

 pable shape, that rose as naturally as the rememl)rance of its 

 original, when conjured by a diseased imagination, became to " the 

 mind's eye " a terrible reality. 



Mr. James A. Whitney read the following paper on 



AGRICULTURAL ENGINEERING. 



One of the most interesting and characteristic results of modern 

 industrial progress, is shown in the almost universal tendency to 

 divide all arts and sciences of a utilitarian nature into distinct pro- 

 fessions or specialities. To cluster around each of the branches 

 thus created, the theoretic information and the practical skill which 

 will best conduce to the special object sought, thereby enabling the 

 wants of every business or occupation to be met by the acquire- 

 ments of men trained and experienced in all its phases. Thus, 

 for instance, the science of cnjxineerinjr first divided into civil and 

 military, has in the course of time developed the other classes of 

 mining and mechanical engineering; and although a proficiencj'' in 



