ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
119 
For instance, tlie total amplifying power of a Microscope with tube 
of standard length carrying a 1-in. objective and a 2-in. ocular should 
be 9 x 6 = 54, instead of 10 X 5 = 50, as per the usual rule. 
It is to be noted, however, that while the formulae here given are 
theoretically correct for the objective and ocular respectively, and are 
applicable to any image distances by substituting the desired imago 
distance for p', as 10 was substituted for it in the examples given, yet 
there are many complications in the practical application of any formula 
to the determination of the actual amplification obtained by the modern 
compound Microscope ; among these complications are, 
1st. The highly complex construction of many objectives, making 
it very difficult to ascertain with any degree of accuracy the position of 
the optical centre, which difficulty is still further increased when the 
objective under consideration is furnished with a correction arrangement 
for various thicknesses of cover-glass, which, by varying the relative 
positions of its component lenses, varies its actual and nominal focal 
length and the position of its optical centre. The exact position of the 
optical centre of the ocular is also, at times, difficult of determination. 
2nd. The refractive condition of the observer’s eye is also a factor 
in the amplification under which the image is finally seen, for the reason 
that the dioptric system of the observing eye becomes, in fact, a part of 
the ocular, and any difference of its refractive power greater or less than 
that required to focus on the retina rays proceeding from a radiant situated 
at the given image distance, must be added to or subtracted from the 
refractive power of the ocular, and thus decrease or increase its focal 
length. That is to say, a person who can and does accommodate for 
precisely 10 in. while looking through the Microscope will, if all 
the other conditions are rigidly complied with, see the image under the 
exact amplification indicated by the formula, while one, who by reason of 
myopia or of excessive use of the muscle of accommodation accommodates 
for a less distance, will see it under a greater amplification, and the 
emmetrope or hyperope who relaxes his accommodation to less than that 
required to bring rays from a radiant at 10 in. to a focus on his 
retina, will see it under a less amplification than that indicated by the 
formula. For instance, let us take the case of the combination of 
1-in. objective and 2-in. ocular for which we have found the total 
amplication, when image distances are taken as 10 in. in case of 
both objective and ocular, to be 9 X 6 = 54. If the observing eye 
be accommodated for just 10 in., the image will be seen clearly and 
under an amplification of X 54. If, however, the eye is accommodated 
for any other distance, then the image will not be clearly seen and a 
change must be made in the adjustment of the Microscope to make 
it clear. The reason is that the excess or defect of the refraction of 
the eye above or below what is required to accommodate it to 10 in. 
has, in effect, been added to or taken from the refractive power of the 
ocular. 
Suppose an observer, as the result of myopia or from spasm of, or 
voluntary action of the muscle of accommodation, accommodates for a 
distance of 5 in. instead of 10 in. ; he has, in effect, added to the 
refractive power of the ocular the refractive power of the lens which 
represents the difference between a refractive power of 10 in. and of 
