In making use of the higher power objectives--from No. 5 on--it should be remembered, that the lenses are corrected for cover glasses of 0,17 mm in thickness and for a microscope tube-length of 170 mm. When using the oil-immersion objectives it is particularly desirable, that this exact tube length should be employed. With a view to facilitate the adjustment of the tube-length the draw tubes of all our larger stands are graduated in millimeters, the scale indicating the exact length of the microscope tube in any given position of the draw tube. In this connection it should be remembered, that the width of the collar of the nose-pieces is 15 millimeters, and that consequently, when a nose-piece is attached to the tube the reading of the draw tube scale should be 155 mm instead of 170 when the adjustment is proper. [Illustration: Figure comparative merits of the dry and immersion systems.] The above sketch may serve to make clear the advantages of the immersion objectives over those of the dry series. It is intended to represent diagrammatically a section through a cover-glass and the front lens of an objective, one half of the figure representing the conditions present in the case of the Oil-immersion, the other those which are present in the dry objectives. It will be noticed that by the interposition of a drop of oil of the same index of refraction as the glass between the cover-glass and the objective the refraction which occurs in the dry system when the light leaves the upper surface of the cover-glass is done away with. Since this second refraction is attended with much loss of light it must be evident, that in the immersion system a much greater quantity of light enters the objective than is possible, other things being equal, in objectives of the dry system. If we let _u_ represent one half the angular aperture of an objective, represented in the diagram by _D'BN_ in the case of the immersion, and _n_ the index of refraction of the medium interposed between the cover-glass and the objective we have in the formula _n_ sin. _u_ a mathematical expression of the optic power of the various systems of lenses, or in other words for what is designated the numerical aperture. The following table gives the numerical apertures of objectives of the various systems and of various angular apertures. In the dry system _n._, representing the index of refraction of air is taken as 1.00; in the immersion systems _n_ equals 1.33 for water, 1.52 for cedar oil and 1.66 for monobromnaphthalin. A glance at the table will suffice to show the great advantage which the immersion objectives have over those of the dry series. --------------+----+----+----+----+----+----+----+----+----+----+----+----+----+---- Angular |10° |20° |30° |40° |50° |60° |70° |80° |90° |100°|110°|120°|130°|140° aperture 2 _u_| | | | | | | | | | | | | | =Numerical apertures.=