108 THe Microscope. 
The most common form used with the microscope is the 
negative, or Huyghenian; another is called the Solid, from the 
fact that it is made of one piece of glass. It was invented by 
Mr. Tolles. In this, when the diaphrdgm is put in, which is not 
always the case, it is formed by cutting a circular groove into 
it, same as in the Coddington lens. These eye-pieces are made 
only in high powers, and although there is a greater loss of light 
by the passage of it through the entire length of the glass, than 
in the Huyghenian, there is less by.refraction, as this occurs in 
only two surfaces, while in the other it is in four, and it con- 
sequently gives a better illuminated field with the same ob- 
jective. 
The method of denominating eye-pieces of different powers 
has been by letters, A, representing the lowest power. This 
method has always been open to criticism, as it insures no uni- 
formity,—and none has been sought after, one eye-piece of one 
maker representing a totally different power from one of the 
same rating of another maker. _A more rational system is com- 
ing into use, and was first introduced in this country. By it the 
eye-pieces are rated according to the equivalent focus of the 
eye-lens. Thus an eye-piece in which the equivalent focus is 1 
inch, is called a1 inch; the } inch in the same manner, and so 
on. This method gives approximately the magnifying power, as 
we know that a 1 inch lens magnifies ten times. It is sup- 
posed by many that the size of the tube influences the size of 
lenses and field, but beyond a certain limit this has no effect. 
The field is never enlarged beyond the requirements of the 
formula. 
It is a pity that when the Royal Microscopical Society 
adopted the standard screw for objectives, it did not endeavor 
to arrive at more uniformity in the size of tubes. Our Ameri- 
can Society of Microscopists is making a laudable effort in this 
direction, as alSo in the nomenclature of eye-pieces. Its com- 
mittee has for the past two years been working up the subject, 
and it will probably be decided upon at the meeting to be held 
in this city. There are two difficulties which arise, one of which 
is the fact that one standard will not be sufficient, as it is mani- 
fest that a large tube which will. be required in a large instru- 
ment will be out of place in a small one, and vice versa. 
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