Professional Advancement Service 



Extended correspondence and much delay are involved in securing a professional 

 position or in finding a man of proper qualifications for a given post. The Professional 

 Advancement Service, has been established to aid the younger men and women in 

 botany and related sciences in securing promptly and inexpensively such positions 

 as they may desire. This Service is also useful to the heads of departments or di- 

 rectors of laboratories, experiment stations or other institutions as a means of making 

 known the vacancies that they wish to fill. The method of the Service consists in 

 publishing in The Plant World a brief notice stating the training and experience of 

 the applicant or the requirements of the vacancy. Each person using the Service 

 is designated by number and his identity may be learned on application to the Editor 

 of The Plant World. A list of eligibles and of open positions is kept, and inquiries 

 are referred to the advertisers who are mentioned or who seem best suited to the 

 requirements stated. The Professional Advancement Service may thus have the 

 opportunity to mention a man for a desirable position or a promotion a number of 

 times during a period of several months. The charge for the service is $2.00 for each 

 case. The notice regarding qualifications offered or desired will be published at 

 frequent intervals, and the aid of the Service will terminate only on request or after 

 the fulfilling of its object. 



THE PLANT WORLD TUCSON, ARIZONA 



LIGHT FILTERS 



We are iible to supply a series of glass screens which absorb or transmit various 

 parts of the spectrum and are useful for experimental work in growth, photo- 

 tropism, and photosynthesis. These glass screens were developed by Dr. D. T. 

 MacDougal, and are now available in bell-jars 36 x 36 cm. in size, with perforated 

 top, or in plates 16.5 cm. square. 



Red Plates. A good monochrome transmitting to .610 u $1.50 each 



Blue Plates. Transmitting wave lengths less than .510 u $1.50 each 



Orange Plates. High transmission in red and green to .530 u. 



Bridges the speqtrum from red to blue $1.00 each 



Heat-Absorbing Plates. Absorb most of the infra-red and 97% 

 of the heat of a Nernst lamp. Give a pyrheliometer reading 

 about half that of good window glass. Transmit 65% of inci- 

 dent white light. In plates only $1.00 each 



Uviol Plates. Transparent to visible spectrum, transmitting 

 ultra-violet to .310 u. in sheets 6 mm. thick to .300 u. in 

 sheets 3 mm. thick. In plates only $1.00 each 



Terms for bell-jars furnished on request. Additional information 



cheerfully given 



THE PLANT WORLD TUCSON, ARIZONA 



