6 HOW TO WO! IK 



diameters. The definition of these glasses should be good, and 

 they should transmit plenty of light. Any lines in a structure 

 examined by them should appear sharp and distinct. The field 

 should be flat, every part of it in focus at the same time, not too 

 small, and there should be no coloured rings round any object 

 subjected to examination. The achromatic object-glasses consist of 

 three sets of lenses, each of which is itself compound, but 

 Mr. Wenham has made some excellent high powers with a single 

 front lens. An important improvement in the making of object- 

 glasses has been recently made by Mr. Wales, of Fort Lee, New- 

 Jersey, who at the suggestion of Prof. H. L. Smith, of Kenyon 

 College, U.S., has added a second posterior combination, which 

 may be substituted for the ordinary one when objects are to be 

 examined with very oblique light. The arrangement also possesses 

 some advantages for photographic purposes. 



Some object-glasses of high power are now made so that the 

 object must be viewed through a thin stratum of distilled water 

 placed between and touching the surfaces of the front lens of the 

 objective and the covering glass (a immersion). The image has a 

 peculiar brightness, and as Mr. Brooke has observed, the object 

 is more highly illuminated, because more oblique rays are admitted 

 than would otherwise pass into the lens ; the working distance of 

 the objective is somewhat increased, while the price of glasses of 

 the same magnifying power is less. Immersion object-glasses were 

 first made by M. Hartnack, of Paris, the successor of Oberhauser, but 

 others, as M. Merz, of Munich, and M. Hasert, of Eisenach, have 

 since produced them. 



The use of objectives of very high magnifying power is discussed 

 in another part of this volume. 



7. Spherical and Chromatic Aberration. Unless the objective is 

 properly corrected for spherical and chromatic aberration, pi. I, 

 figs. 5 & 6, it is valueless to the observer. Spherical aberration 

 may be known by the want of sharpness when a fine line 

 or small spot, or body with a well-defined circular outline is ex- 

 amined. Instead of the lines appearing sharp and distinct and 

 definite, they seem to be blurred and foggy, even when focussed with 

 the utmost care, and when there are several lines or spots near to 

 one another, they appear to run together, producing a general 

 shadow, instead of each one being distinctly defined and separated 

 from its neighbours. If the glass has not been properly corrected for 

 chromatic aberration, lines are seen with coloured fringes, blue if th 

 lens is under-corrected, reddish if over-corrected. 



8. Flatness of Field can be tested by moving an object from one 



