Mr. Howarp Gruss on Great Telescopes of the Future, 5 
In this will probably be found the secret and key to the whole difference of 
opinion. A veteran and well-known worker with Refractors declared ‘he never 
looked into a Reflector without drawing away his eye in disgust;” and workers 
with Reflectors cannot understand how the Refractor workers can bear that 
dreadful fringe of colour from the secondary spectrum. The same applies to other 
matters. Newtonian observers cannot understand how those who observe with 
Refractors or Cassegrain Reflectors can bear to strain their neck so in looking up 
through the tube ; while the Refractor and Cassegrain workers cannot understand 
how the Newtonian workers will break their backs sitting or standing bolt upright, 
when they might be reclining comfortably on an easy chair as they do. After all, 
when this comes to be investigated it resolves itself into but little more than a 
question of to which telescope the observer has been most accustomed. Each 
observer becomes in time wedded to his own instrument ; he has done his work 
with it, the credit of his discoveries is due to it, and he naturally falls into the 
idea that no other can be as good. 
As regards the effect of the arm of the small mirror on the image, I do not 
think much advantage can be claimed in this respect. I use a small central mirror 
supported on a thin arm for illuminating the micrometer field in all our achromatic 
telescopes, and although I provide the means of instantaneously removing this in 
case the observer may think the perfection of the image is injured, I do not find 
that this is made much use of. 
6th.—Less effect from air currents in Refractors. 
The Refractor having a tube closed at both ends, and the Reflector being open 
at upper end, the condition of air currents is quite different in the two cases, to the 
disadvantage of the Reflector, for in it the upper end being open, there is nothing to 
prevent currents of hot and cold air up and down the tube, and in and out of the 
aperture, and for this reason great advantage has been found in ventilating the 
tubes, 7.¢., making it of some open-work construction in order that the air may 
pass through and across and remove currents of differing temperatures. This diffi- 
culty is not felt with Refractors ; but, curious to say, in the largest Refractor at 
present in existence (the Washington 26-inch), Professor Newcomb informs me 
that considerable inconvenience is felt sometimes from the outside of the object 
glass cooling down more quickly in the evening than the inside, which produces a 
decided effect on the spherical aberration and injures temporarily the otherwise fine 
definition. He consequently recommends the use of lattice or ventilated tubes 
for very large Refractors. If this be found necessary, this advantage of the 
Refractor vanishes. 
I now come to the second part of my subject, viz., the advantages that Reflectors 
possess over Refractors. 
