2o8 Brigade^Major Kater on the Light 



was then placed in the end of the tube, and an aperture was 

 made, which was enlarged by cutting out one circle after an- 

 other till the card appeared equally bright through both teles- 

 copes, and of this, the eye judges most accurately. 

 The following measures were then taken. 



Cassegrainian Telescope. 

 Diameter of the circular 



opening in the paste- inch. 



board - - 2, 70 



Diameter of the back of 



the small mirror - 1, Op 

 Length of the arm - 0,805 

 Thickness - - 0,200 



Gregorian Telescope. 



Diameter of the large inch, 



mirror - - 3, 90 



Diameter of the back of 



the small mirror - 1, 00 



Length of the arm - 1, 45 



Thickness - - 0, 20 



From the above measures the following calculations were 

 made. 



Cassegrainian Telescope. 



Inch. 



Area of the circular open- 

 ing in the paste-board 5,726 



Area of the back of the 

 small mirror to be de- 

 ducted - 0,933 



Area of the arm to be de- 

 ducted - - 0,l6l 



■1,094 



Area of the portion of the 

 mirror exposed to the 



light 



4,632 



Gregorian Telescope. 



Inch. 

 Area of the large mirror 1 1,946 



Area of the back of the 

 small mirror to be de- 

 ducted - 0,785 



Area of the arm to be de- 

 ducted - 0,290 



1,075 



Area of the portion of the 

 mirror exposed to the 

 light - - 10,871 



a.. 



