formed by the objective. B / A'. The inverted virtual image, a projection of the 

 retinal image. 



Axis. The optic axis of the microscope and the eye. 



Cr. Cornea of the eye. L. Crystalline lens of the eye. R. Single, ideal, refract- 

 ing surface at which all the refractions of the eye may be supposed to take place. 



F. The principal focus of the positive ocular. F r . The principal focus of the ob- 

 jective. 



Mirror. The mirror reflecting parallel rays to the object. The light is central 



\ 42). 



Pos. Ocular. An ocular in which the real image is formed outside the ocular. 

 Compare the positive ocular with the simple microscope (Fig. 4). 



Fig. 6. Hg. Ocular. Huygenian ocular showing the general character of a neg- 

 ative ocular, and the action of the field and eye-lenses. (From Carpenter, after 

 A. Ross). 



B B. Blue image, convex to the eye-lens, that would be formed if no field-lens 

 were present. 



R R. Red image, convex to the eye-lens, that would be formed but for the pres- 

 ence of the field-lens. B B and R R show also that the objective is over-corrected 

 for the blue rays, as the blue image is formed farther from the objective than the 

 red image. As blue rays are more refrangible than red, the image would naturally 

 he nearer the objective than the red image. 



W W R' R'. Blue and red real images as actually formed under the influence of 

 the field-lens. Both are concave to the eye-lens, and " as the focus of the eye- 

 lens is shorter for blue rays than for red rays by just the amount of the difference 

 in the place of these images, their rays, after refraction by it, enter the eye in a 

 parallel direction, and produce a picture free from false color." The field-lens 

 also aids in rendering the field flat. 



EL. Eye-lens. FL. Field-lens. 



Fig. 7. Sectional view of a Huygenian ocular {Hg. ocular), to show the forma- 

 tion of the Eye-Point. 



Axis. Optic axis of the ocular. D. Diaphragm of the ocular. E L. Eye-lens. 

 F L. Field-lens. 



E P. Eye-point. As seen in section, it appears something like an hour-glass. 

 When seen as in looking at the ocular, i. c, in transection, it appears as a circle of 

 light. It is at the point where most ra3 - s cross. 



