At the present time, Professor Lowell states that this "curious triangulation has been traced over almost every portion of the planet's surface, whether dark or light, whether greenish, ochre, or brown in colour." In some parts they are much closer together than in others, "forming a perfect network of lines and spots, so that to identify them all was a matter of extreme difficulty." Two such portions are figured at pages 247 and 256 of Mr.Lowell's volume.
_The Oases._ The curious circular black spots which are seen at the intersections of many of the canals, and which in some parts of the surface are very numerous, are said to be more difficult of detection than even the lines, being often blurred or rendered completely invisible by slight irregularities in our own atmosphere, while the canals themselves continue visible.

About 180 of these have now been found, and the more prominent of them are estimated to vary from 75 to 100 miles in diameter.

There are however many much smaller, down to minute and barely visible black points.

Yet they all seem a little larger than the canals which enter them.

Where the canals are double, the spots (or 'oases' as Mr.Lowell terms them) lie between the two parallel canals.
No one can read this book without admiration for the extreme perseverance in long continued and successful observation, the results of which are here recorded; and I myself accept unreservedly the substantial accuracy of the whole series.