with reference to fig1 now in case the entrance angle of the light a into the objective side surface 10b is denoted by u03b1 the entrance angle of the light b into the same surface is denoted by u03b2 the entrance angle of the light c into the same surface is denoted by u03b3 and the critical angle on the objective side surface 10b is denoted by u03b8 1 u03b1 lt u03b8 1 sin 1 n 3 n 2 therefore the light a will be refracted by the objective side surface 10b and will pass to the side of the objective 5 next when the critical angle on the object side surface 10c is denoted by u03b8 2 if the entrance angle u03b2 satisfies the relation that u03b8 2 gt u03b2 gt u03b8 1 the light b will be totally reflected in the objective side surface 10b and will be refracted on the object side surface 10c a part of it will proceed as a light b 1 into the water enclosed between the surface 3 of the object to be observed and the object side surface 10c of the prism and the other part will be flennel reflected on the object side surface 10c will become a light b 2 and will again enter the objective side surface 10b further if the entrance angle u03b3 satisfies the relation that u03b3 gt u03b8 2 the light c will proceed while being totally reflected on both of the objective side surface 10b and object side surface 10c and as already explained will be finally reflected by the reflecting.