ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 495 



two beams which, although having been both reflected in the thin 

 lamella, have undergone diffraction by the net, one at its entrance into 

 the lamella, and the other at its emergence. These fringes can be 

 rendered more brilliant by increasing the reflecting power of the lower 

 surface, e.g. by employing a metallic surface, a condition unfavourable 

 for New T ton's rings. The fringes can be used with white light for con- 

 veniently verifying the form of the lower surface, whether it be of glass, 

 of metal, or of mercury. 



Entoptic Vision and the Entoptiscope.* — If a pinhole perforation 

 through a card, or similar opaque object, be turned towards the light 

 and held close to the eye, a circular disk is seen, which is the shadow cast 

 by the circular aperture of the iris. By such means small opacities in 

 the path of the rays in the eye are projected on the retina and become 

 visible. This method of self-examination of obscurities within the eye- 

 ball is termed entoptic. A casual experiment of this kind led W. F. 

 Barrett to the unwelcome discovery that he had incipient cataract of 

 both eyes. The entoptiscope is an instrument which he has invented 

 for viewing, delineating, and measuring entoptic objects. PI. XVII., 

 fig. 1, shows the first form of the designer's entoptiscope. It consists of 

 a pair of vertical brass pillars supporting a head rest, which can slide 

 from side to side so as to bring either eye vertically over the pin-hole 

 contained in the revolving diaphragm of the eye-pieee. This diaphragm 

 has pin-hole apertures varying in diameter from 0*1 to 2*5 mm., 

 and a pair of pin-holes each " 1 mm. diameter, and 2 mm. apart, 

 so that by revolving the diaphragm either a single aperture of any 

 given size, or a double aperture, can be successively brought before the 

 eye. Below the pin-hole eye-piece is a transparent scale divided into 

 fractions of a millimetre ; the shadow of this scale falls upon the eye of 

 the observer, and is thence projected, much magnified, upon the ground- 

 glass stage below, along with the shadows of any opacities seen in the 

 eye. At the base of the instrument is a concave mirror, which can be 

 adjusted so as to illuminate the eye-piece brilliantly, using the light of 

 the sky or that of a lamp. A sharply-pointed and hard pencil is used 

 by the observer to trace the image seen on the ground-glass stage. The 

 image of the pupillary disk, the projected shadow of the opacities in the 

 eye, and the pencil point, are all seen in the same plane with perfect 

 clearness. After the drawing has been made the ground-glass can be 

 removed and photographed for future comparison. A later form of the 

 instrument is shown in PI. XVII. fig. 2. The vertical pillar P is 

 hinged so that the observer may incline it to suit himself ; a single 

 pillar is used so as to leave the hand free to draw on the ground-glass 

 stage G, which carries a supporting hand-rest R. The eye-pieces E E 

 have cups shaped to fit the eye and bring the cornea within a definite 

 distance of the pin-hole. In this way the pin-hole can be placed at the 

 anterior focus of the eye (above half an inch from the cornea), and the 

 stage is placed at a fixed distance so as to give a definite magnification. 

 It is important that the observer in using the entoptiscope should be 

 comfortably seated and completely at ease ; he should have his hands 



* Scientific Proc. Royal Dublin Soc, xi., Nos. 7-8, March and May 1906. 



