"iJ. Ap?iiT,Tj7o.'] I^oyal Mieroscoincal Societij. 187 



adding by the arrow-head any number from ^th to ^:f'^ths, undula- 

 tions can be retarded, which include all the colours of the spectrum ; 

 and I think the simphcity of this arrangement will be self-evident 

 to all. 



I do not know of any method in which the number of quarters 

 in use can be seen so plainly as in this : it is no use to have thirteen 

 quarters in the Selenite films unless we can have them under com- 

 mand, to know what suits our object best. Messrs. Beck's arrange- 

 ment is well suited for this form of cell {ivhicli I think is all that is 

 necessary), and the achromatic condenser can be used without diffi- 

 culty. But in almost all the new methods of late, toothed wheels 

 have played an important part ; and I have added six to mine, the 

 object of which is to rotate any number of quarters simultaneously. 

 They are put in gear by a small milled head at the side. 



The instrument, including the cutting of the teeth of the wheels, 

 having been done by me (an amateur), it will be a sufficient apology 

 for the way in which the brasswork is finished. 



