300 THE AMERICAN MONTHLY TOct 



With the achromatic the greatest sharpness of the image 

 hitherto formed was limited to one color, or two at the 

 most, of the light transmitted (yellow or perhaps yellow- 

 green for visual lenses and blue-violet for those used in 

 photography), while all the other rays of the spectrum 

 gave more er less confused images, appearing as colored 

 images (fringes surrounding the sharp image), and part- 

 ly as a general haze spread over more or less of the entire 

 field. With the new system, however, the images are 

 nearly, if not absolutely, sharp for all colors, hence the 

 the quality of the finite image whether visually or photo- 

 graphically considered is independent entirely of any special 

 color of the illuminant. It is easy therefore to understand 

 the great value of these lenses in defining difficult struc- 

 tures and how it is that they give such sharp and excel- 

 lent photographs, let alone such white and colorless im- 

 ages of objects like diatoms. Again too, Professor Abbe 

 pointed out that in achromatics color correction was only 

 obtained for one zone of the objective, the others being 

 more or less defective, but that in his apochromatic sys- 

 tem the chromatic aberration was corrected equally in all 

 parts of the field. That affords the reason why the re- 

 sulting definition is so fine because the image produced by 

 each color is said to perfectly coincide with that of the rest. 

 But there are yet two other points of interest which 

 must be mentioned and they are : — (1) That owing to all 

 these perfections mentioned so much more light is obtain- 

 ed. Dr. Dallinger has stated that an ordinary achro- 

 matic of the best type would only pass 140 parts, but an 

 apochromatic, 225 out of a possible 300. (2) That also 

 owing to the perfection of their performance the apo- 

 chromatics permit the use of eyepieces of extraordinary 

 power without producing what is technically known as a 

 "rotten image." An achromatic will only permit the use 

 of an eyepiece having at the most the initial magnifica- 

 tion of about 4 to 6, but so perfect is the performance of 



