54 PHOTOMICROGRAPHY 



as their corrections are altogether superior to those 

 found in achromatic objectives, Prof. Abbe intro- 

 duced the term " Apochromatic " to distinguish them. 

 Spherical aberration is corrected in them for two colours 

 instead of one, as in achromatic objectives, and three 

 colours are brought to a common focus instead of two, 

 which so reduces the distance between the foci of the 

 various parts of the spectrum that secondary spectrum 

 is almost eliminated. These corrections can, moreover, 

 be accurately made for all zones of the objective. Their 

 construction is, however, more complex than that of 

 achromats of corresponding power, and their cost very 

 considerably higher. 



Semi- Apochromatic and Fluorite Objectives. Improve- 

 ments in the corrections of achromatic objectives are 

 continually being made, mainly by the introduction of 

 the new glasses and fluorite in their construction, with 

 a consequent recomputation of the components. Several 

 makers call the lenses so produced " Semi-apochro- 

 matic," others use such terms as " Improved achro- 

 matic," and " Fluorite " to describe them, or give them a 

 special name, such as " Holoscopic." They really are 

 improved achromats, and have not the complete correction 

 of the apochromats, but, on account of the high trans- 

 parency of fluorite and the more perfect corrections 

 made possible by the use of that mineral and the new 

 glasses, these lenses approach very closely to apochro- 

 mats in working properties, although only chromatically 

 corrected for two colours, and yet, on account of their 

 comparatively simple construction, they are produced 

 at much lower prices than the more complicated apo- 

 chromatic objectives. 



Under-Correction of High-Power Objectives. High-power 

 objectives of wide aperture (p. 160), both apochromatic 

 and achromatic, have an uncorrected front lens almost 

 hemispherical in shape, and exhibit an outstanding defect 



