ERECTING EYE-HECK. 45 



that of Hartnack about 10 diameters, according to Frey ; that of 

 Tolles, according to Hage/n, 1 7 to 50 diameters and upwards a 

 quite superfluous magnifying power. 



d. The Erecting Eye-piece. 



As the inverted position which the compound Microscope gives 

 to images increases the difficulty of preparing objects on the stage, 

 even if it does not retard research, modern opticians have en- 

 deavoured to effect a reinversion of the image katoptrically or 

 dioptrically. The simple Microscope, which is generally used for 

 dissections, ought to be abandoned for this purpose and be replaced 

 by a better instrument, which would give the very important 

 advantage of a greater focal distance. This idea has been realized 

 as follows : 



To the ordinary eye-piece two other lenses were added, which 

 were inserted in the microscope-tube with the convex sides 

 upwards. The whole eye-piece is thus identical in arrangement 

 with that of the terrestrial telescope, and therefore causes a rein- 

 version of the objective-image. Instruments with these eye-pieces 

 are to be obtained from Ploessl, of Vienna, of two different models, 

 of which the smaller gives a linear magnification of 70 150, the 

 other up to 300. Increase of the magnifying power is here 

 obtained simply by drawing out the eye-piece tube. 



Another method, which Oberhieuser was the first to make use 

 of, consists in employing a complete compound Microscope of low 

 magnifying power as an eye-piece, bringing it so near to the 

 objective that the real image formed by the latter is viewed by it. 

 An object-distance is thus obtained, which admits of the freest 

 use of dissecting instruments, &c., with moderately high magnify- 

 ing powers. On the other hand, the distinctness of the images 

 leaves much to be desired in comparison with that obtained 

 with a simple Microscope. Such dissecting Microscopes are 

 supplied by Hartnack, the successor of Oberhteuser ; they are 

 provided with a rotating stage and give a magnifying power of 

 10100 diameters. 



Lastly, the inversion of the image may also be attained by 

 reflecting prisms, which are applied either above the eye-piece 



1 " Archiv fur mikr. Anat." Bd. vi. p. 215. 



