According to the search results, the total magnification of a microscope using a 40x objective lens and a 10x eyepiece is 400x. This is because the total magnification is a combination of the magnification of the objective lens and the eyepiece. To calculate the total magnification, you simply multiply the magnification of the objective lens (40x) by the magnification of the eyepiece (10x) . It is important to note that there are limits to the amount of total magnification that can be reached before empty magnification occurs, which is when the image continues to be enlarged, but no additional detail is resolved. To avoid empty magnification, it is important to consider the numerical aperture of the objective lens when selecting a combination of eyepieces and objective lenses for optimal magnification.