MICRO-ANASTIGMATS 79 



power under the same convention. This confusion 

 is made all the worse by the various tube lengths, 160, 

 170, 200, and 250 mm., employed by different makers, 

 and unless all the factors are known the power engraved 

 on the eyepiece, or the value given in the catalogue 

 for the letter or figure used in its place, cannot be used 

 as the basis of an estimation of the magnification pro- 

 duced when using objectives and eyepieces by different 

 makers. Two eyepieces of the same focal length may 

 be called by different makers X 10 and x7, and 

 therefore the practical worker is driven to make up his 

 own table of magnifications, unless he uses one make of 

 lenses only. 



Small Photographic Objectives. Many makers con- 

 struct small lenses on the lines of photographic anastigmats 

 of I", 2", and 3" focal length (25, 50, and 75 mm.), fitted 

 with the R.M.S. screw for microscopes. To these they give 

 special names, such as Planar, Micro-Summar, Holostig- 

 mat, etc. They are largely used for low-power photo- 

 micrography, and have an advantage in flatness of field 

 and covering power over ordinary microscopic objectives. 

 They are used without eyepieces, and should be chosen 

 for general views of large objects, for which their magni- 

 fication is sufficient. Their aperture is given in the 

 " f " ratio notation commonly used for photographic 

 lenses, denoting the ratio of the aperture to their focal 

 length, and forming a convenient measure of their 

 rapidity. 



Comparison of "/" Ratio and N.A. 



N.A. : 



2fx (m+1) 



is a convenient formula for converting " f " ratios into 

 terms of N.A. ; in it m= magnification. 



If, as a concrete case, the " f " aperture of one of 



