73 



Transit Solar Attachment. 



For running Meridian or other lines bj the San. 



Written for this catalogue with special reference to the wants of Public Land Surveyors, for both common and 



mineral lands, by J. B. DAVIS, Assistant Professor of Civil Engineering, 



University of Michigan. 



1. Remarks. The attachment herein referred to is the Davis and Berger 

 solar screen, prism, and colored shade glass, used for direct solar observation. 

 These inventions have been devised by the Mr. Berger, of the firm of C. L. Berger 

 & Sons, and by the writer. They are simply for the purpose of enabling one 

 to make an observation directly upon the sun's centre. This observation being 

 secured by readings of the horizontal and vertical circles, is reduced so as to 

 give the direction of the line of sight of the transit at the instant of the observa- 

 tion. Thus knowing the direction of the line of sight at a given instant it becomes 

 simply necessary to turn off the angle which this line of sight makes with the 

 meridian, to ascertain the position of the meridian. This angle is what is obtained 

 by reducing the observation, as above mentioned. A brief reference to the history 

 of these devices will best explain them. It occurred to the writer to see if an image 

 of the sun could be formed behind the eye-piece of a telescope at the same time 

 an image of the cross-wires was, and the latter image be made to quarter the 

 former, by allowing the sun to shine into the object end of the telescope and thence 

 directly through it. The experiment was made by holding a piece of white paper 

 behind the eye-piece, and adjusting the focus of the eye-piece and object glass. 

 The very first trial was readily successful. The next thing was to see if the posi- 

 tion of the instrument could be located by this means as near as the circles would 

 read. By the same simple means it was soon found that a motion given to the 

 telescope by either tangent screw might be so slight that the eye ct>uld not detect 

 it upon the circles, but evidence of it would be apparent in the position of the images 

 with reference to each other. This fact at once settled the question of whether 

 this would be a sufficiently delicate means of observation. It showed that the 

 observations would be closer than the circles would read. After some trials and 

 some months rest these facts were brought to the notice of others, and finally were 

 submitted to Mr. Berger for his opinion. He made a screen which the writer 

 exhibited at the first annual convention of the association of Michigan Engineers 

 and Surveyors at Lansing. The matter was further studied by Mr. Berger. The 

 screen was much improved, and the mechanical construction of it brought to 

 the standard of the worjt done by this firm. Mr. Berger soon conceived the 

 idea of making the screen of ground white glass in a brass frame, as shown in 

 figs. 1 and 2, so one might observe the position of the images directly upon it, 

 and thus secure not only the comfort of an easy position in observing, but the 

 consequent accompanying accuracy. The arm of attachment was perfected from 

 time to time. The screen of ground glass is mounted upon an arm that admits of 

 all adjustments of position, and is so attached to the side of the telescope tube that 

 it can t>e turned up out of the way when not needed. The reflecting prism 

 can be screwed on to the eye-piece cap for observing at high altitudes. This also is 

 adjustable so as to look in any desired direction from the telescope tube. The 

 diagonal eye-piece also has its movable colored shade glass as above stated. With 

 these attachments observations on the sun at all altitudes may be made in two 

 ways. By looking directly at it through the simple colored glass for low altitudes, 

 or through the prism and its shade glass for high altitudes. The other way is to 

 receive on the screen the images of the cross- wires and the sun and make the image 

 of the cross-wires just quarter the image of the sun by means of the slow motion 

 screws to the circles of the instrument. For this method the colored shade glasses 

 are not to be used. With this complete outfit one may work whichever way seems 

 best. 



