390 Lord Blythswood and Dr. Marchant on 



§ 4. Method of Using tJw Grating. 



The apparatus having been set as described in § 2 (Method 

 of ^Mounting) , tHg .echelon-mount and -the .whole instrument 

 -having been carefully levelled, the line which it is required 

 to examine is set om the slit S 3 of_tbe echelon-mounting. 

 The echelon having been slippedinto position and adjusted as 

 described, several possible conditions may be observed in the' 

 eyepiece. - — -— — ~- 



Taking first the case of aline which is not split up under 

 the resolving power of the echelon, there may be one bright, 

 single line visible, together with (if the light be very bright)- 

 a series of lines gradually diminishing in intensity as the] 

 distance from the central line is increased ; or there may be 

 two lines of equal intensity with other lines gradually dimi- 

 nishing in intensity towards the edge of the field; obviously 

 there are numerous other intermediate conditions possible, in 

 which one line is brighter than the other. The lines of 

 gradually diminishing intensity are the successive orders of 

 the grating for this particular light. 



The position of the grating in which two equal lines or 

 orders are visible in the field may be described as that giving 

 the double-order condition. The corresponding position in 

 which only a single line or order is visible may be described 

 as the single-order condition. 



By turning the grating slightly round its central vertical 

 axis it will be fouiidthat, say from the single-order position, 

 another order will appear to one side of the single line, and 

 gradually increase in intensity until the two lines or orders 

 are equal: if the- turning be still further continued, the 

 original line will fade und the new line will get brighter, and 

 at the same time will move towards the centre of the field 

 until we have the original single-order condition with the 

 new order. '"." . ~_ 



It will be found, when the echelon is turned from a position 

 slightly tilted in one direction to a position slightly tilted in 

 the other direction, that there is a certain position in which, 

 whatever way the echelon be turned, the order on the left- 

 hand side, say (in the eyepiece), will always brighten. In 

 any other position, of course, by tilting the echelon one way 

 the order on the right of the original brightens, while an 

 opposite tilt will brighten the order on the left. This con- . 

 dition corresponds to the position of the echelon in which the 

 plates are normal to the incident light. 



It will also be found that when the position of the echelon 

 described above (which may be called the direct position) has 

 been obtained, a tilt in one direction will rapidly diminish the 



