Microscopes for crystallographic and petrological research are now specially constructed for measuring the angles of crystals. Erector eye-pieces and erecting prisms are employed for the purpose of causing the image presented to the eye to correspond with that of the object. They are also helpful in making minute dissections of structure; the loss of light, however, by sending it through two additional surfaces is a drawback, and impairs the sharpness of the image. Nachet designed an extremely ingenious arrangement whereby the inverted image became erect; he adapted a simple rectangular prism to the eye-piece. The obliquity which a prism gives to the visual rays when the microscope is used in the erect position, as for dissecting, is an advantage, as it brings the image to the eye at an angle very nearly corresponding to that of the inclined position in which the microscope is ordinarily used. The Achromatic Objective. [Illustration: Fig. 110.--Pan-aplanatic Achromatic Objectives.] _The Achromatic Objective_, of all the optical and mechanical adjuncts to the microscope, is in every way the most necessary, as well as the most important. The ideal of perfection aimed at by the optician is a combination of lenses that shall produce a perfect image--that is, one absolutely perfect in definition and almost free from colour. The method resorted to for the elimination of spherical and chromatic aberration in the lens has been fully explained in a former chapter. It will now be my endeavour to show the progressive stages of achromatism and evolution of the microscope throughout the present century. It is almost as difficult to assign the date of the earliest application of achromatism to the microscope as to that of the inception and many modifications of the instrument in past ages; indeed, the question of priority in every step taken in its improvement has been the subject of controversy.