a. What is the instrument shown above?

b. What additional optical modification is usually incorporated 
    for ophthalmic uses?

c. Supposing the the objective lens is 40D, L is 20cm and the eyepiece 
    is 10D. What is the total magnification?

 

Answer to aA compound microscope.
 

Answer to b A Porro's prism.
The image is inverted both horizontally and vertically.
A Porro's prism is used to invert the image to make it upright.
 

Answer to c 20X 
A compound microscope uses a very short focal length objective lens 
to form a greatly enlarged image. The image is then viewed with a short
focal length eyepiece used as a simple magnifier. The following formula can be used to calculate the magnification.
M = mo (me)
M = total magnification
mo = linear magnification = L/fo (focal length of the objective lens)
     = 0.2/0.025
     = 8
me = angular magnification = Fe/4
     =  10/4
     = 2.5
M = 8(2.5)
    = 20