308 SCIENTIFIC APPARATUS. 



This form of construction, recently adopted in excellent instru- 

 ments made by Fuess at Berlin, was contrived by Dr. V. von Lang, 

 when on the staff of the British Museum, as an improvement on 

 one previously employed there ; and to that crystallographer is 

 also due the means of adjusting the relative positions of the tele- 

 scope and the crystal along the direction of the axis of the instru- 

 ment, by moving, not the telescope, but the crystal and its 



attendant apparatus of adjustment, which slides along and can be 

 clamped to an accurately turned cylinder of steel, forming the 

 prolongation of the axle of the goniometer. Adjustments for 

 regulating the parallelism of the plane containing the axis of the 

 telescope and the signal, with the plane of the goniometer's disc, 

 and for correcting errors in all other parts of the instrument, 

 render it, without doubt, more or less complex in use ; but, when 



