12 HOW TO WORK 



observer as one, and in relief. There is, however, no true stereoscopic 

 image, for the one picture seems to be in every respect, save in 

 intensity of illumination, the counterpart of the other. 



I have examined many objects by the arrangement of Messrs. 

 Powell and Lealand, and find that it works exceedingly well in prac- 

 tice, and is less fatiguing than the monocular plan, and I can recom- 

 mend it to those who work with very high powers. 



Modifications of the principle adopted by Messrs. Powell and 

 Lealand in their binocular for high powers, have been suggested by 

 Mr. Wenham, with the view of utilising some of the light lost in their 

 system, but I have not had an opportunity of carefully comparing 

 the working of Mr. Wenham's prisms with those of Powell and 

 Lealand. From Mr. Wenham's description there appears to be some 

 difficulty in obtaining perfectly satisfactory results. " The two 

 prisms need not be pressed into contact if so, Newton's rings are 

 formed they may be set a visible distance asunder, but great care 

 is needed in adjusting the small prism so as to get both reflections 

 combined, otherwise a blurred image will be seen in the slanting 

 body." Mr. Wenham, however, assures me that the results are highly 

 satisfactory if the instrument is properly made according to the 

 directions he has given. 



New binocular microscope. More recently, Mr. Tolles, of Canas- 

 tota, New York, has adapted a binocular eye-piece to the ordi- 

 nary single body. This gives a large field well illuminated, and 

 seems to perform well with low and medium magnifying powers. 

 Professor H. L. Smith, in a note to Dr. Maddox, to whom I am 

 indebted for the following observations, says he has even used it 

 with the T V and ^ objectives. 



It is constructed thus : an adjustable shallow achromatic erector 

 or eye-piece slides in a setting that fits the tube of the single body 

 microscope. By this an image is formed at the eye-glass end. This 

 image then passes through the flat surface of an equilateral prism 

 placed over the eye-lens, and by it is bisected, one half being 

 refracted towards the right, the other half to the left. After these 

 rays have emerged from the prism, they pass into prisms of the form 

 used by M. Nachet in his binocular microscope (and suggested for 

 use here by Professor Smith, as the rectangular prisms first employed 

 by Mr. Tolles, did not give satisfactory results), and escape from 

 their outer surfaces at the angle of total internal reflection. The 

 rays are lastly transmitted through two deep eye-pieces or oculars 

 superposed over the two prisms last described. See pi. V, fig. 17. 



By a small pinion these prisms are adjusted for the variable 

 distance between the eyes of different observers, and Mr. Ladd 



