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 



