4 18 Dr. J. Stokes on some Optical Pfuenomena. 



stinct image inverted apparently on the other side. When 

 I moved the pin downwards, the image moved upwards ; and 

 vice versa, showing that it was inverted. Wiien the hole was 

 oblong instead of circular, I found that if the pin was placed 

 in a direction perpendicular to the shorter axis, the image as- 

 sumed the appearance of i in the oblong delineated in fig. 2. 

 When it was very oblong, and the pin placed perpendicularly 

 to its longest axis, the image was like two indistinct lines. If 

 it is in the form of a long rectangle, nearly the same phasno- 

 mena occur. But in that of an equilateral triangle, the image 

 always appeared like indistinct lines. 



In fig. 3. let i represent a small lens, p a pin placed be- 

 tween it and the point f its focus, and e a minute hole in a 

 card placed beyond the pointy' at h. When all these things 

 were in the above position, I looked through the lens and saw 

 a magnified and pretty distinct image of the pin beyond the 

 minute hole, and bordered with a bright fringe of light. The 

 image is represented in the figure by the two concentric cir- 

 cles i and ss\ the annulus between being that fringe. When 

 I moved the card further from the focus, the image became 

 larger and more distinct ; and when the pin was moved nearer 

 to that point, it became smaller and more confused. The image 

 was erect: but when both the pin and the hole were placed be- 

 tiioeen the focus and lens, it became inverted and confused ; and 

 the same phaenomena followed on the removal of the lens : and 

 when they were both beyond the focus, the image was erect, 

 but greatly confused. 



When the diameter of the pin-hole is increased to more than 

 one-fifth of an inch,all phaenomena cease to be presented by it. 



A very peculiar kind of telescope may be constructed from 

 a moistened piece of silver paper and a minute hole in a cir- 

 cular piece of card both fitted up in a tube. It is represented 

 in fig. 4, where an inverted and magnified image of the ob- 

 ject r.r is depicted on the paper ss', and transmitted from 

 thence to the eye at e. The image is confused. 



The sun may be looked at through this instrument without 

 the intervention of smoked glasses. 



When ^ p\s increased to the length of several feet, sa' con- 

 tinuing stationary at four inches, if the observer looks through 

 it at the sun, he will appear of increased size and of an oval 

 form. An increase of the magnifying power (by increasing 

 sp') makes it more elliptical. A deci'ease makes it less until 

 at length it reassumes the circular form. 



Dublin, Sept. 25, 1H;29. JoMN StoKF.S. 



