126 SCIENCE PROGRESS 



to take the key off the glass table. She saw it clearly through 

 the glass, high above her diminished head. But Alice was not at 

 the end of her resources, nor is the astronomer. Alice reduced 

 herself still further and crept under the door and he may 

 further reduce the light of the stars and so see between them. 

 This time he uses a dark glass, the action of which is at once 

 apparent when the nature of the images is considered. 



They are brightest at the centre and surrounded by fainter 

 interlacing rings which can well be dispensed with. The tinted 

 glass at once cuts off the light of the rings. It also dims the 

 central image equally all over so that only the brightest part in 

 the middle remains visible. The two middle points of the star 

 images are now seen neatly separated by the gap which 

 previously was filled with the light of their outer edges. So 

 the observer has achieved his purpose in an unexpected way 

 by reducing the light instead of increasing it. 



This digression may appear at first sight to have little to 

 do with Mars but it is not irrelevant, for in the telescope 

 the disc of the planet is made up of an indefinite number of 

 luminous points each behaving in exactly the same way as the 

 two star discs first investigated. It is therefore easily seen 

 that the same methods must be used in separating the several 

 points upon his surface. 



One might at first suppose that the process might be con- 

 tinued indefinitely. But a limitation is set by the apparent 

 brilliance of the surface, because to see clearly the eye requires 

 a certain minimum of illumination ; above this minimum the 

 method may be applied whose importance has long been un- 

 accountably overlooked by many observers. 



In the light of these facts it is easy to see that aperture plays 

 at best a secondary part in planetary observation, which is 

 restricted by the climatic difficulties by which we are so greatly 

 hampered on our earth. 



Experience in many observatories has convinced me that 

 as yet there is not one which is so highly favoured in a matter 

 of climate as that of Lowell at Flagstaff, Arizona. At this 

 station (at an altitude ol a mile and a half above sea level), not 

 only is the air very steady and clear but there is actually less of 

 it and that only the best part left over the observer's head. 



Here is then the best place to determine the limits of useful 

 aperture in planetary observation and the result to which 



