118A 



aary to take a reading on a point very near the instrument, and has to resort to variou. 

 means (all of them inaccurate) of getting around the difficulty. The attachment men- 

 tioned consists of a small aluminum tube containing a simple lens, which is attached in 

 front of the objective. The lens is so placed in the tube that it can be accurately 

 centered by means of 4 adjusting screws. The effect of this lens is of course to bring 

 rays to a focus nearer to the objective, and thus enable the observer to focus a nearer 

 object than would otherwise be possible. When the telescope will focus no nearer than 

 6 feet, the attached lens, marked 1, is ground so that it will focus objects 6 feet away 

 when the objective tube is drawn away in. This allows the entire motion of the focusing 

 slide for distances between 6 and 4 feet. For distances nearer than 4 feet a second 

 lens may take the place of the first and will focus up to about 2} feet. If the two are 

 used at once the distance is reduced to about two feet. 



With this pair of lenses there is no distance between two feet and infinity at which 

 objects cannot be focused. The accuracy of work done with this attachment is in no 

 way affected by the centering of the attached lens itself, as this is capable of perfect 

 adjustment. The only way in which error can occur is through the imperfection of 

 the objective tube. If the cylindrical surface of the object-head of the telescope on 

 which the attachment is placed is not concentric with the optical axis of the teles- 

 cope this error will enter into the adjustment of the attached short focus lens. This 

 error, however, is never large on an instrument sent out by our firm. But even 

 admitting that there may be some error here, it must be rememembered that this lens 

 is never used for objects more than about 6 feet away ; consequently the resulting 

 error on the point is entirely negligible, and the convenience of the attachment in 

 many cases is so great that it entirely outweighs any such consideration, since the 

 work done at this distance will be entirely consistent with the work done with the 

 instrument on the longer distances. The attachment fills a want that has long been 

 felt by engineers and is certainly a step in advance in the perfection of instruments of 

 precision. 



To attach this device to their old instruments it will be necessary to send the instru- 

 ment to them, as every lens attachment must be specially fitted and centered. How- 

 ever, it can be supplied with any of their new instruments, either Transits or Levels, 

 made since 1899. 



When attached to transits, No. 1 permits focusing objects to about 3| feet. No. '2 

 permits focusing objects to about 24- feet : both permit focusing objects to about 2 feet 

 from center of instrument. . 



This is so important a feature that one trial will convince one that it is indispensa- 

 ble to the outfit of an engineer The device is patented. The Messrs. Berger 

 are also prepared to attach it to their Wye and Dumpy level, for focusing nearly as 

 close as stated above for transits. For prices see catalogue, page 169. 



