THE EYEPIECE OR  OCULAR The eyepiece is considerably simpler in its construction than the

objective . Its purpose in a microscope is by means of narrow cones of rays to represent at infinity the real magnified image which the objective produces . As, however, the object represents a real image, the problem is to project a transparent diapositive . It is therefore impossible to observe this image through an ordinary lens . Since many of the rays coming from the exit-pupil of the objective would not reach the eye of the observer at all, it is necessary, in order to make use of all of them, to direct the diverging rays forming the real image so that the whole of the light enters the eye of the observer . This is effected by a collective lens; it may be compared with the second part of the condenser system of a projecting lantern . The two most customary eyepieces consist in two simp:e piano-convex lenses, whose distance one from the other is equal to half the sum of the two focal lengths . One of these is the Ramsden eyepiece (fig . 37) . If the real image produced by the objective coincides with the collective lens, only the inclination of the principal rays is altered, the form of the cone being affected only to a very small extent . The lens nearer the eye, which has about the same focal length as the collective lens, is distant from it by about its focallength . The eye-lens converts diverging pencils into parallels .

Both lenses together form the exit-pupil of the objective behind the eye, lens, so that this image, the exit-pupil of the

total system or the Ramsden circle, is accessible to the eye of the observer . It is possible to see the whole field through this pupil by slightly moving the head and eye . In practice the real image is formed not directly . r Lz L2 =collective-, L3 = eye-lens . DD =diaphragm of the field of view . P"P"= Ramsden's circle, or exit-pupil of whole microscope . on the collective lens but a little in front of it, because otherwise all the particles of dust on the collective would also be seen magnified . In the other type, the Huygenian eyepiece (fig . 38), which is much more widely used, the collective lens is in front of the real image ; it alters the direction of the principal rays and somewhat diminishes the real image . In this type the eye-lens is about twice as powerful as the collective lens, and makes the rays parallel . Here also the exit-pupil is accessible to the eye and through it the whole field can be seen by moving the head and eye . In both eyepieces micro-meters or cross-wires are used for measuring in the plane of the real L2 = collective-, L3 = eye-lens . DD =diaphragm of the field of view .

P"P" = Ramsden's circle, or exit-pupil of whole microscope . image . The Ramsden eyepiece is the most convenient for this because this plane lies in front of the collective lens, and the objective image has not yet been influenced by the eyepiece . As both eyepieces are used with very small apertures (about f : 20) no

attempt has been made to overcome the spherical aberrations, which are usually very slight; neither, as a rule, are the eyepieces chromatically corrected, care has only to be taken by a suitable choice of the distance of one lens from the other, that the coloured images derived from a colourless object should have the same apparent size . Since, however, the difference of chromatic magnification cannot be overcome in powerful objectives, this error is still further increased by the eyepiece . The difference of chromatic magnification cannot even be over- come in apochromats, and to cancel this aberration Abbe devised the compensating ocular (fig . 39) . The weak compensation oculars resemble a Huygenian eyepiece with achromatic eye-lens, whilst the more powerful ones are of a different construction . These eye-pieces are intentionally provided with a different chromatic magnification, which however is in opposition to that originating in the objective . They have also a shorter FIG . 39.-Compensating Eye-focus for red, and a longer one for pieces (Zeiss) . blue, and thus magnify the red image more than the blue; and as the objective gives a large blue and a small red image, the two cancel one another and a colourless image is produced .

These eyepieces are very convenient in use, for when they are changed the

lower focus always falls in about the same plane . In German and French microscopes the optical length of the tube, when apochromats and compensation-eyepieces are used, is 18o mm . By multiplying the magnification of the objective by the number L3 on the eyepiece the total magnification of the microscope is obtained . By the magnification of the objective is meant the ratio of the distance of distinct vision to the focal length of the objective . As powerful achromatic objectives show differences of chromatic magnification in the same way as apochromats, compensation eyepieces can be used in combination with these objectives .