Flat-Wavelet Resolution. 593 



account of that substitution of tan /3 for sin /3 which is referred 

 to on p. 50o of the Phil. Mag. for December J 896, where /3 

 is the small angle to which the letter ft is assigned in the 

 figure on p. 586 above : and partly because the iris diaphragm 

 and stops under the condenser of a microscope are always 

 situated farther down than is desirable. The construction of 

 the instrument makes it impossible to bring them up into 

 coincidence with the principal focus of the condenser. Now 

 it is an image of these and of the brightness bounded by 

 them which fixes the apparent position of the concentration 

 image as we see it : on which account each punctum of that 

 image as seen by the observer is the concentration, not of 

 light that has passed up through the air or oil space under 

 the objective as a strictly parallel beam, but of light that was 

 slightly convergent when it traversed that space. These two 

 circumstances cause the concentration image to be a repro- 

 duction of the central part of the indicator diagram which 

 is slightly distorted, but the distortion is so slight that it 

 falls short of what could be detected by the unassisted eye 

 and it causes no real embarrassment in making even delicate 

 experiments. 



46. The transition from the microscope to the spectroscope 

 is an easy one. In fact, part of the apparatus which has 

 to be put together to make a microscope, is essentially a 

 spectroscope — viz. : the stop under the condenser, the con- 

 denser itself, the microscopic object on the stage, and the 

 objective. To see this, employ as the microscopic object a 

 ruling of equidistant parallel lines, such as one of those bands 

 of lines (ranging from 5000 up to 120,000 in the inch) 

 which have been exquisitely ruled on the under side of 

 cover-glasses by Mr. H. J. Grayson of Melbourne ; place 

 under the condenser a stop in which there is a single narrow 

 slit parallel to the band of lines; and then look through the 

 " blank " eyepiece at the concentration image which is 

 formed near the back (t. e. uppermost) lens of the objective. 

 In this concentration image will be seen a simple image of 

 the slit — the zero spectrum, as Lord Rayleigh well designates 

 it — attended on either side by spectra of the first and 

 succeeding orders which are produced by the ruling under 

 examination. In fact, the condenser has become the colli- 

 mator of a spectroscope and the objective is its telescope. 

 And, as in this case. BO with the ordinary types of >pect,ro- 

 scope. rli»- spectrum, a- seen in the telescope, is essentially a 

 concentration image, each visible punctum of which is the 

 concentrated light of one of the little sheafs of u f'w's into 



