Apparent Size of the Moon, 381 



ciently satisfactory to convince those who are sceptical in the 

 matter. The best illustration brought forward as yet, to my 

 knowledge, was that which appeared some time since in ' Recrea- 

 tive Science/ in an article upon " The Rising and Setting of 

 the Sun Physiologically considered." The writer had, one dark 

 night, been gazing for some time upon the glowing furnace of a 

 lime-burner, after which, on going out into the dark, he saw the 

 luminous image of the fire in the air before him as a large circle ; 

 but happening to look up, this image apparently shrank up to 

 about half its former diameter. This, however, does not appear 

 to satisfy many persons; and since your most interesting lecture 

 of the 16th ult. was the means of my discovering a far more 

 forcible and complete illustration of this singular effect, I ven- 

 ture to trespass upon your time by detailing it to you. On my 

 return home after that lecture I proceeded to make sundry 

 experiments upon binocular vision, amongst which were the 

 following:- Figl rigZ 



Centring the eyes upon a 

 distant point A, I introduced a , t 



stereograph of the moon at such ,' ' ; 



a distance from the eyes that I i ( \ / 



the axis of each passed centrally ; \ " f" ~ B 



through the picture taken for _J_ J^.b \ / 



it; the result was a perfect and « * 7 c 



beautiful stereographic projec- 

 tion of the moon at A, appa- ,' ', / ► 

 rently solid and spherical (as 

 fig. 1). Then, without in any 

 way altering the distance be- 

 tween the eye and the slide, ! » 

 I centred the eyes upon a ** ** 

 point nearer to me than B, in 



such a way that the axis of the right eye passed through 

 the centre of the left picture, the left also passing through 

 the right : the result was a perfect view of the moon, but in 

 this case inside out, as you termed it (the moon appearing quite 

 concave), the image being seen in the air at C, and appearing 

 in a most perceptible degree smaller than when in the former 

 case I saw it apparently at A, nearly double the distance. Now 

 as the distance between the real object and the eye was in both 

 cases identically the same, the image upon the retina must have 

 been of the same size in each experiment, the apparent enlarge- 

 ment of the object in one case and contraction in the other being 

 purely mental. This singular effect may have been noticed by 

 yourself before this ; but in case it should not have come under 



