A fact in practical optics, calculated to affect some observations rather seriously, has been discovered within the last few years. It is this: When a white body is viewed on a dark ground, its size is exaggerated by some illusion of our vision; and, on the contrary, a dark body seen on a bright ground appears smaller than it would were the ground of a dark color, differing from that of the body only as much as is required to render them distinguishable. Now, in the transit, a dark body is seen on an intensely bright ground. It becomes necessary, therefore, to bring in a correction which will compensate for the error arising from this optical illusion. This has been done by Stone, who studied out the whole matter, arrived at certain modes of correction, applied them to Encke’s calculation, and maintains that the true result of the observations of 1761 and 1769 should be 91,730,000 miles. Thus all seem to agree that the sun’s distance must be less than 92,000,000 miles, and that Encke’s estimate was too great by 3 or 4 per cent. This is the stage at which our astronomers now take up the question, and aim to obtain a yet more definite and precise result. Will they succeed? They are full of confidence now; what they will say after their observations we may know a year hence. Some of our readers may like to know what is the course followed in making the observations and in calculating the results. We will give a slight account of the chief points, sufficiently detailed to enable one with an ordinary knowledge of trigonometry to understand how the conclusion is reached. The astronomers will follow two methods, known as those of Halley and of Delisle. They each require two suitable stations, so far apart on the surface of the earth as to give a satisfactory base‐line. In fact, the further apart, the better, all things else being equal. For Halley’s method, the two stations lie as nearly north and south as may be. For Delisle’s, they lie east and west.