88 BINOCULAR MICROSCOPES [Ch. Ill 



is more satisfactory to have the magnifier held by some mechanical 

 means like the focusing holder shown in fig. 19. 



The advantage of using both eyes has led to the production of 

 binocular arrangements to be held in place by a band around the head. 

 These are necessarily rather expensive. 



The needs were stated to Dr. A. C. Durand in 1913, and at my 

 request he devised a pair of spectacles which magnified approximately 

 1.5 diameters. In addition to the magnifying curve he added the 

 correction for astigmatism, and combined these corrections and curves 

 with a 4 degree prism, base in, to " relieve the excessive convergence 

 which would otherwise be necessary with such short focus spheres." 

 With these spectacles, which are much cheaper than any device on 

 the market, and which have all the corrections needed for the eyes of 

 the individual observer, it is very easy to carry on minute dissection. 

 The prisms serve to prevent the weariness which comes so soon with 

 great convergence. The eyes look nearly straight ahead, as in viewing 

 distant objects, and are therefore in position of rest. These spectacles 

 have also been found of much service in reading proof of fine print. 



Experiments with Binocular Microscopes 

 § 146. Erecting, double-objective binocular. — Put a pair of ob- 

 jectives of 40 to 50 mm. focus, and a pair of oculars, 4X or 5X, in place. 

 The oculars are put in place in the ordinary manner (§ 45), but the 

 objectives are now often mounted in a sliding objective changer per- 

 manently. To put them in place one has simply to slide the pair in 

 the mounting into the proper groove at the lower end of the micro- 

 scope body. 



Place the microscope where a good light can be had and put on the 

 stage some transparent specimen like an organ with the blood vessels 

 injected or with both the blood and the lymphatic vessels injected. 

 Reflect the light up through the specimen, and focus. 



§ 147. Arranging the microscope for binocular vision. — Until one 

 has had some experience with binocular microscopes it is not easy to 

 tell whether one is seeing with one eye or with both. In order to see 

 with both eyes it is of course necessary that each eye should receive 

 the beam of light from its own ocular at the same time, and this can 



