focus the eyepiece carefully upon the cross wires, then to re- 

 produce an image of the field on this plane with the objective. 



Turn the telescope away from the sun and toward the sky, or 

 simply hold a piece of white paper inclined in front of the objective 

 so as to throw a diffused sky light into the telescope. Focus the 

 eyepiece with the worm slot until the cross wires appear distinct. 

 1'uiallax is produced by improper focus and creates the effect of 

 unstable conditions in the position of the wires. 



To test for parallax, after focusing the telescope on some fixed 

 point, move the eye up or down, or from side to side, over the 

 range of vision and see if the w ire seems to leave the point. If so, 

 it means that the images of the field and the cross wires are in two 

 slightly separated focal planes. Depending upon whether the 

 object seems to follow the movement of the eye, or move contrary, 

 shows whether the image is beyond, or behind, the plane of the 

 cross wires. In any event the objective will have to be slightly 

 readjusted until the phenomenon is overcome. 



As the work progresses through the day, one's eyes will be- 

 come tired and a readjustment of the eyepiece will be necessary. 

 We provide a wide range of focus in all of our oculars to fit all 

 such inequalities of vision. 



IV. To Secure a Position for the Cross Wires Normal 

 to the Axis of Revolution. 



The cross wires, in reality, are spider lines taken fromthecocoon 

 of the Eperia Diadema. They are mounted at right angles to 

 each other and fastened to the diaphragm with shellac. This ad- 

 justment seeks to secure the proper position of one wire. If the 

 vertical wire, for instance, is vertical, the horizontal wire will be 

 horizontal. Proceed as follows: 



Select some point at convenient range and set the vertical wire 

 upon it. With the lower plates clamped, move the telescope up and 

 down with the tangent screw and note the result. If the wire adheres 

 to the point, its position is correct, but if the point appears to pass 

 from one side to the other, release the tension in all four diaphragm 

 screws, and placing the blade of the screw driver against one of the 

 washers tap gently in the direction desired and test as before. 



It is not necessary that the plates should be leveled to perform 

 this test, but following the process in tost I it would be customary 

 to have them so, and in this event the vertical wire may be tested 

 against a plumb line. 



This test may also be conducted by moving the instrument 

 from side to side on the vertical axis and watching the contact of a 

 certain point with the horizontal \\ire, as suggested for leveling 

 instruments on \< 



