Ch. IX] PAR- FOCAL OCULARS AND OBJECTIVES 309 



less, as the field-lens makes the objective image smaller (fig. 23-24, 

 183). In the case in hand the reduction of the objective image was 

 j, so that the real image of the objective with the iield-lens in place 

 was 15 diameters, and of the whole microscope then only 150 diameters, 

 as the magnification of the eye-lens is unchanged. But as the objec- 

 tive is not changed in power by, the field-lens, the effect of the 

 entire ocular, field- and eye-lens, must be the entire magnification of 

 the microscope divided by the power of the objective, which is 20 

 diameters. 150 -j- 20 = 7.5. In this case then the ocular as a whole 

 magnifies the real image of the objective (20) 7.5 times. In all Huy- 

 genian oculars, then, the field-lens acts as a reducing lens, the eye- 

 lens as a magnifier, and this is true whether the microscope is used as 

 in ordinary observation (fig. 23-24) or for projection (fig. 183). 



§ 479. Magnification of drawings. — In determining the magnifi- 

 cation of a drawing made with a camera lucida or with projection- 

 apparatus, by far the best method of determining it is to remove the 

 specimen and put in its place a stage micrometer and project the 

 image of the micrometer upon the drawing paper. Make a few lines 

 of the micrometer image and indicate the value of the spaces (fig. 

 103), then at any time one can determine exactly what the magnifi- 

 cation is (§ 276). 



§ 480. Par-focal Oculars. — By this is meant oculars of different 

 power in which the microscope remains in focus on changing the oculars. 



As originally constructed the microscope had to be focused every 

 time the oculars were changed. Mr. Edward Pennock, in seeking to 

 overcome this inconvenience, wrote to Professor Abbe for advice in 

 1 88 1. After successfully producing oculars of different powers for 

 the Acme microscopes of James W. Queen & Co., according to the 

 directions given by Professor Abbe, Mr. Pennock, as editor of the 

 Microscopical Bulletin and Science News, published in Vol. Ill, 1886, 

 pp. 9-10, the following, with Professor Abbe's letter: "Changing 

 Eye-pieces without altering Focus," etc. " Some years ago the writer, 

 in looking up certain questions in connection with eye-pieces, took 

 occasion to write to Professor Abbe, and his reply, kindly given, is so 

 clear and to the point, and of such interest and value, that we take 

 the liberty of publishing it for the benefit of our readers." 



