This test may be made still more delicate by using the observing telescope, _a_, at as low an angle as possible, thereby bringing out with still greater effect any error that may exist in the surface under examination, and is the plan generally used by Alvan Clark & Sons. Another and very excellent method is that illustrated in Fig. 2, in which a second telescope, _b_, is introduced. In place of the eyepiece of this second telescope, a diaphragm is introduced in which a number of small holes are drilled, as in Fig. 2, _x_, or a slit is cut similar to the slit used in a spectroscope as shown at _y_, same figure. The telescope, _a_, is now focused very accurately on a celestial or other very distant object, and the focus marked. The object glass of the telescope, _b_, is now placed against and "square" with the object glass of telescope _a_, and on looking through telescope a an image of the diaphragm with its holes or the slit is seen. This diaphragm must now be moved until a sharp image is seen in telescope _a_. The two telescopes are now mounted as in Fig. 2, and the plate to be tested placed in front of the two telescopes as at _c_. It is evident, as in the former case, that if the surface is a true plane, the reflected image of the holes or slit thrown upon it by the telescope, _b_, will be seen sharply defined in the telescope, _a_. [Illustration: FIG. 5.] If any error of convexity exists in the plate, the focal plane is disturbed, and the eyepiece must be moved _out_. If the plate is concave, it must be moved _in_ to obtain a sharp image. Irregular errors in the plate or surface will produce a blurred or indistinct image, and, as in the first instance, no amount of focusing will help matters. These methods are both good, but are not satisfactory in the highest degree, and two or three important factors bar the way to the very best results. One is that the aberrations of the telescopes must be perfectly corrected, a very difficult matter of itself, and requiring the highest skill of the optician. Another, the fact that the human eye will accommodate itself to small distances when setting the focus of the observing telescope. I have frequently made experiments to find out how much this accommodation was in my own case, and found it to amount to as much as 1/40 of an inch. This is no doubt partly the fault of the telescopes themselves, but unless the eye is rigorously educated in this work, it is apt to accommodate itself to a small amount, and will invariably do so if there is a preconceived notion or bias _in the direction of the accommodation_. [Illustration: FIG. 6.]