444 Mr. W. Hardic's Description of a New Pseudoscope, 



this effect is not so ludicrous as might at first be expected, being 

 unaccompanied with anything like unequal or local distortion of 

 the features. 



Of course these effects of alteration of distance are necessarily 

 accompanied (as will be shown when the principle of the instru- 

 ment is explained) with corresponding variations of apparent 

 magnitude, or lateral extension in the field of view. This is best 

 exemplified by a square card, which, when viewed perpendicularly 

 at a certain distance, appears of its true form and dimensions ; 

 but when turned upon its diameter without changing its mean 

 distance, the side which approaches the observer diminishes, 

 while that which recedes enlarges ; and these changes combining 

 with the more rapid elongation of the other two sides, convert 

 the square into a long trapezium seen rather more obliquely 

 than the card really is, and with its smallest side next the 

 observer. Thus it appears that when an object approaches, its 

 breadth and height are diminished ; when it recedes, these are 

 increased. Beyond and within certain distances, however, these 

 changes of size as well as the exaggeration of depth cease, and 

 the object is seen double ; because binocular combination is ren- 

 dered impossible in the one case by the visual rays from the 

 same point entering the eyes convergently, and in the other by 

 their divergence increasing beyond what the eyes can accommodate 

 themselves to by squinting. 



Another effect which is produced by both forms of this pseu- 

 doscope is, that the size and distance of the binocular image 

 never both at the same time correspond with those of the object. 

 If the sizes appear the same, the distance of the image seems 

 greater ; if the distances agree, the size of the image is less. This 

 effect, however, so far at least as the increase of distance is con- 

 cerned, is independent of the binocular principle of the instru- 

 ment, and may be seen with one eye alone ; it is merely inci- 

 dental to the construction, and arises from the reflected visual 

 ray being necessarily longer than the straight line drawn between 

 the eye and the object. 



The above binocular effects are all the results of an increase of 

 the difference between the right and left retinal images. That this 

 is an adequate cause for them, and how it operates, will be 

 readily understood by those already familiar with the phseno- 

 mena exhibited by the stereoscope, and their explanation. The 

 same effect of increased d.epth is seen with that instrument when 

 the binocular combination of the two drawings is produced under 

 a diminished angle of the optic axes. In both cases it results from 

 a similar alteration of the natural relation between the axial angle 

 and the difference of the retinal images. The mode in which this 



