102 ELEMENTARY TECHNOLOGY. 



proved by Holland's Triplet, with a diaphragm in front of 

 upper lens. Further improvement was made by the Steinheil 

 loup (as it is called) and the Briicke lens. The former is an 

 achromatic doublet, and the latter an addition to it of a con- 

 cave eye lens which magnifies more the further it is drawn 

 from the loup or doublet. . Simple microscopes are used as 

 hand lenses, or as dissecting or preparing microscopes. 



2. Chromatic aberration is due to the unequal refrangibility 

 of the colored rays composing white light. Thus the violet 

 ray V, Fig. 5, PL I, is focused nearer the lens than the red 

 ray R, and the other rays of the spectrum are intermediate. 

 Different sorts of glass vary in the proportion between refrac- 

 tive power and the dispersion of the colored rays, hence it is 

 possible to calculate curves of such ratios as shall overcome 

 chromatic aberration when lenses of different kinds of glass are 

 combined, as in telescopes. Newton's failure, Euler's hint from 

 the eye, Dolland, etc. 



Achromatic objectives for the microscope aim to overcome 

 both spherical and chromatic aberration. Generally triplets, 

 often complicate; different formulae. Wenham's formula con- 

 nects four convex lenses by one concave of flint glass. This 

 has reduced the price of good lenses. 



Opticians usually aim to unite the extreme red and violet 

 rays, but, on account of the irregular proportion of refraction 

 of colored rays by different glass, the best objectives have 

 some imperfection. Professor Abbe and Zeiss, after years of ex- 

 periment, obtained new glasses and constructed objectives 

 which unite not only the extreme or mean rays, but nearly all 

 in one beam of white light. They are called Apochromatic 

 lenses. Fluorite is also used in their construction. 



5. Effects of cover glass. 



Microscopic objects require a cover glass for preservation 

 and for the safety of the objective, but its use produces aber- 

 rations easily seen with high-power objectives. An objective 

 which defines well without a cover will be imperfect if a cover 

 be used. 



In Fig. 6, PI. I, rays from a luminous point, or object, are 



