* On Vision. 129 



unpolished glass at the eye end of the instrument, that even 

 its colour, a deep red, was very distinguishable. But, after 

 the lens was taken away, the shadow of the line was so 

 broad, faint, and indistinct, as not to be perceived by a 

 person unacquainted with the experiment : and what might 

 have been easily expected, this faint shadow was accom- 

 panied with a camera obscura picture of remote objects, 

 though dark and ill defined. 



The following experiments show how the eye is adjusted 

 to distinct vision in a very satisfactory manner. These were 

 made with a transit telescope, which has an object-glass of 

 2|- inches aperture, and 3-i- feet focal distance, adjusted to 

 observe celestial objects. 



I directed this telescope to an object at the distance of 40 

 yards. This object consists of a circular hole -^ of an inch in 

 diameter made in an iron plate, with a plate of metal painted 

 white placed at some distance behind it. With the whole 

 aperture of the object glass, this hole in the iron plate is 

 invisible ; but when the aperture is contracted to -l of an 

 inch, the hole appears so distinct, that it is easy to see when 

 it is bisected by the wire in the focus of the object glass, to 

 less than -^^ of an inch ; but remote tent- strial objects 

 viewed with the same aperture are seen very imperfectly. 



Hence we see the reason of the pupil's contracting when 

 we at-tentively view a near object*, and why it expands when 

 we look at those that are remote. 



That the iris is the only organ by which the eye is ad- 

 justed to distinct vision, may be clearly understood by the 

 following experiments : 



Experiment I. — Let a remote object be observed through 

 an aperture of about ^^ of an inch in diameter, made either 

 in a thin piece of metal or a card, and if the object be seen 

 in a strong light it will appear as distinct as to the naked 

 eye, though not so bright. Then introduce a small object 

 in aline between the remote object and the eye, at the di- 

 stance of six or eight inches from it, and these two objects 



• This property of the eye has long been known. See Dr. Turin's Essay 

 on distinct and indistinct Vision, in Dr. Smith's Optic«, vol. ii. p. 138. 



Vol. 3 i . No. 122. July 1 808. I will 



