lo Experiments on the Relative Intensities 



field. No inconvenience arises from that circum- 

 stance ; on the contrary, several advantages are de- 

 rived from that arrangement. 



Instead of the screen just described, I sometimes 

 make use of another, which differs from it only in this, 

 that the hole in it, which determines the form and 

 dimensions of the field, instead of being quadrangular, 

 is round, and Iyo inches in diameter. And, when this 

 screen is made use of, the shadows are increased in 

 width (by means which will hereafter be described) in 

 such a manner as completely to fill the field, appear- 

 ing under the form of two hemispheres, or rather half 

 disks, touching each other in a vertical line. The 

 object I had in view in reducing the field and the 

 shadows to a circular form was this : I imagined that 

 by diminishing the number of objects capable of act- 

 ing upon the mind, and particularly by removing all 

 straight lines and angles and all unnecessary varieties 

 of lights and shades, the attention might be concen- 

 trated and fixed in such a manner as to render the 

 sense of sight peculiarly acute in distinguishing any 

 difference in the simple objects presented to the eye. 

 But, however plausible this reasoning may appear, I 

 own the experiment did not answer my expectation. 

 It is true the apparent densities of two equal hemis- 

 pheres of shade, in contact with each other, may be 

 compared with great facility, and when no discernible 

 difference is to be perceived between them it is more 

 than probable that they are in fact very nearly equal ; 

 but still I have found by experience that two equal 

 parallelograms of shade, in contact with each other, 

 may be compared with the same ease, and, I have 

 reason to think, with equal certainty, and that even 



