1889.] MICROSCOPICAL JOURNAL. 277 



object-glass of the microscope on to the focusing screw (and subse- 

 quently the dry-plate) in the camera. For high-power work a sub- 

 stage condenser will be required, but at first a i-inch objective will be 

 high enough while experience is being acquired by the beginner. 



EDITOKIAL. 



A New Piece of Apparatus. — Dr. Lighton has for some time been 

 experimenting upon dark-field illumination, and has realized his highest 

 expectations in the piece of apparatus described in this number. With 

 characteristic liberality he at once makes his invention public and gives 

 its use to the fraternity. He explains his method of getting such illum- 

 ination when using lenses of high power and wide aperture. He also 

 has drawn the sketches from which the frontispiece is worked up. Dr. 

 Lighton is one of the most enthusiastic microscopists in the West. 



Royston-Pigott, M. D., the eminent microscopist, died on Sep- 

 tember 14th, at Eastbourne. It will be remembered that he did much 

 work in the improvement of microscopic objectives. It was in recog- 

 nition of this that in 1873 he was elected Fellow of the Royal Mic- 

 roscopical Society. 



Slides Received. — We return thanks to the donor for the following 

 interesting slide : 



Honey (genuine), showing crystals. Mounted shortly after the ma- 

 terial was taken from the hive. Prepared by John Aspinwall, Barry- 

 town, N. Y. 



NOTES. 



A Simple Formula for Finding the Magnifying Power of a 

 Compound Microscope. — Let M = magnifying power, A = the 

 equivalent focus of the eye-piece, B = the equivalent focus of the ob- 

 jective, O = the optical tube length (measured from the anterior prin- 

 cipal focus of the eye-piece to the posterior principal focus of the 



objective) and D the distance for distinct vision; then M= - . — 



A B 

 Edward M. Nelson, in English Mechanic, Oct. 25, 18S9. 



Amplification Required to Show Tubercle Bacilli. — When 

 properly stained and prepared, the bacillus tuberculi can be readily 

 recognized with a good " one-fifth " objective and a bt two-inch " eye- 

 piece, normal tube length, or, roughly speaking, an amplification of 

 350 diameters. We do not think that it could be done much below this 

 amplification, though the sharpness of vision of the observer, his ac- 

 quaintance with the object, and the excellence of his objective would 

 be important factors in settling the question. A one-quarter objective 

 with a two-inch eye-piece, normal tube length, gives an approximate 

 amplification of 200 diameters. 



To be seen and diagnosed for certain, the bacillus tuberculi, in urine 

 or water, must be prepared for examination by following the well-known 



