E. M. NELSON ON BINOCULAR MICROSCOPES. 375 



the Wenhani ; the circles in W and M represent the pupil of 

 the eye, the semi-circle in W is the Ramsden disc in a Wenham, 

 and the portion of the circle in M is the Ramsden disc when 

 the inter-ocular distance is less than the inter-pupillary in the 

 Mercer method. It can at once be seen that a slight movement 

 of the head will not affect the luminosity in W, but in M the 

 head cannot be moved in the slightest degree without either 

 increasing or diminishing the amount of the Ramsden disc 

 cut off by the iris of the pupil ; necessarily, therefore, if in one 

 eye the Ramsden disc is enlarged it is cut off in the other eye, 

 and vice versa, which is the cause of the nickering previously 

 mentioned. A moment's consideration will show how this defect 

 in the Mercer method may to a certain extent be minimised. 

 Obviously the larger the Ramsden disc the less noticeable will be 

 this defect. This, of course, points to the use of a low-power 

 eye-piece with any given objective. The low-power eye -piece 

 has an additional advantage -viz. that the rays emerge at a 

 smaller angle than in the case of a deep eye-piece, and this 

 permits the eye being held at a little distance from the proper 

 eye-point, where the Ramsden disc is expanded. Hence the 

 rule for stereoscopism with the new binocular is to make the 

 inter-ocular distance somewhat less than the inter-pupillary, and 

 not to use eye-pieces deeper than 1| inches, and to hold the eye 

 a little way behind the eye-point. 



There are two other sources of eye strain and fatigue common 

 to all binoculars of whatever type : the first is non -coincidence 

 of the superimposed fields. This by no means uncommon fault 

 is due to carelessness in fitting and putting together ; it is a 

 source of great eye strain and fatigue, and the purchaser of a 

 binocular microscope should be particular to see that the fields 

 are precisely superimposed. The second is a difference of foci 

 in the tubes. In the binoculars both of Messrs. Leitz and Beck 

 provision is made for this by a focusing arrangement in one of 

 the eye-tubes. In the Greenough it is accomplished by means 

 of a focusing adjustment in one of the objectives. If, therefore, 

 a microscope is provided with some such arrangement, the user 

 need not be troubled about this point. 



Passing on now to the second attribute of a binocular viz. 

 that of increased apparent magnifying power, it is found to be 

 as obvious in a microscope as it is in a field glass. Its precise 



