115 



inches, and always to use the longest that the eye-hole will ad- 

 mit of." 



I may say that to facilitate calculations I have made my box 

 twenty-one inches in length, being just the sixth part of the focal 

 length of my speculum, which is ten feet six inches. 



I may also mention one or two things that should be observed 

 in making use of this apparatus. In the first place the dyna- 

 mometer should be laid as close on the eye-glass as possible, other- 

 wise we cannot be certain that our measure is correct. Also we 

 must measure our distance not from the card to the eye-lens, but 

 should the eye-piece be a Huyghenian or negative eye-piece we 

 must measure from the card to the diaphragm half way between 

 the field-lens and the eye-lens, for this diaphragm is or ought to 

 be at the exact focal distance of the instrument and where the 

 image is formed. Should the eye-piece be a "Ramsden," or a 

 positive eye-piece of any kind, of course the measure must be 

 taken from the card to the point where the image is formed, just 

 in front of the field-lens. These precautions being attended to a 

 very exact measure of the magnifying power of any telescope may 

 be obtained. 



I might remark in conclusion that very many persons have an 

 idea that in using a telescope, and especially if it be a refractor, 

 that they are looking through the instrument, but the truth of 

 the matter is that we cannot see through any telescope whether 

 it be a refractor or a reflector, and we see not the object itself, 

 but only a magnified image of that object, which image is formed 

 in the focus of the eye-lens. 



