192 SCIOPTICON MANUAL. whose flatness of field in the table microscope leaves noth¬ ing to be desired in that direction, are entirely unsatisfac¬ tory when used in the gas microscope. One of the most influential causes of this we shall notice presently, but we will here only remark that, as the result of a large experience, we have become convinced that one must be contented with a moderate amount of success in this direction, and not expect what is, at present, at all events, impossible. The second great defect that we encounter in the use of the microscopic lens for projection, is the irregularity of distribution of light upon the screen. By reason of this we may have a field of light with a small bright area at the centre, rapidly fading off into dark¬ ness, with no well-defined margin. The causes of this are, among others, the confusion or want of accurate concentration of the cone of rays from the condensers, and the smallness of the objective, causing it to cut off oblique or marginal rays more or less, according to their obliquity. To remedy this difficulty we can work in two directions. In the first place, we may improve the spherical correc¬ tion of the condensers, or the concentrated character of the source of light. The first of these improvements has already been carried to its practical limit in the best sort of condensers, and the second involves the use of the electric light or of sunlight. In the second place, any increase in the diameter of the microscopic lenses, without a corresponding increase in their actual length, insures a great gain as regards the equal illumination of the field. With this view alone, therefore, a simple uncorrected or single corrected microscopic lens, such as accompanies the regular gas or solar microscopic attachment made for the