ESL / EFL Quiz 175

Eyeglass wearers requiring simple lenses with a spherical power can use the microscope with or without their glasses, provided that the diopter setting of the focusing eyepiece is sufficient. However, if you require lenses with a toric power, for example lenses that refract light differently in the horizontal and vertical then you should wear your glasses for microscopy, since your eye has unsymmetrical aberrations that cannot be compensated by the diopter setting alone. This is how you test your glasses: view a simple geometric figure, such as a circle or a square, through the glasses you have removed from your eyes (see Figure 3). First you hold the glasses horizontally (Figure 3(a), then vertically (Figure 3(b) and 3(c)). If the figure appears compressed or expanded by this rotation about 90 degrees (Figure 3(b)), you will know that you are wearing lenses with a toric or another non spherical power.