34 ERECT AND INVERTED IMAGES [Ch. I 



really the image of the aperture of the objective, and, as shown later, 

 the study of this image enables one to detect lint and other particles 

 on the upper lens of the objective (§ sya). 



The eye-point is also known as the Pupil of the lens; Ramsden 

 Disc or Circle; Lagrange Disc. 



§ 57a. As pointed out by Wright (p. 93), a study of the eye-point with a 

 magnifier gives very definite information and guidance on several important 

 points : 



(i) The aperture of the light in the objective, and hence whether the dia- 

 phragm of the condenser is opened the right amount. 



(2) The centering of the condenser. 



(3) The presence of dust or other opacities on the back lens. 



(4) The partial unsealing of any of the objective combinations. 



(5) The presence of air bubbles in the immersion liquid. 



§ 58. Erect and inverted images with the microscope. — By 



glancing at fig. 2, 6 it will be seen that with the simple microscope 

 the retinal image is inverted; that is, the arrow is turned end for end. 

 In Hke manner the retinal image of any object seen with the naked eye 

 is also inverted (fig. 5). 



On the other hand, with the compound microscope, the retinal image 

 is erect (fig. 3, 20); that is, the arrow points in the same direction as 

 the object. This is because the eye does not see the object directly, 

 but the real image formed by the objective, and this is inverted. 

 From the crossing of the rays on entering the eye, this inverted real 

 image is reinverted, and thus gives an erect image on the retina. Now 

 as objects or their images do not seem to be on the retinal screen, but 

 out in space in the direction of the light rays entering the eye, it is 

 very evident that if the Hght rays are traced from the retinal image to 

 the object or to a virtual image, this will appear to be erect when the 

 image on the retina is inverted, and it will appear inverted when the 

 retinal image is erect, because of the crossing of the rays in passing 

 the pupil of the eye (fig. 2, 3, 6, 20) on their way to the retinal image, 

 or on their way from the retinal image to the apparent position of the 

 object or the virtual image. 



