the first substituent present in the starting cellulose ether is not particularly limited . examples of suitable starting cellulose ethers include a hydroxyalkyl cellulose , e . g ., hydroxyethyl cellulose or hydroxypropyl cellulose ; a carboxyalkyl cellulose , e . g ., carboxymethyl cellulose ; an alkyl cellulose , e . g ., methyl cellulose or ethyl cellulose ; and a cationic hydroxyalkyl cellulose . a hydroxyalkyl cellulose is preferred for obtaining particularly outstanding effects of the present invention . the polymerization degree of the starting cellulose ether to be used in the present invention is generally from 100 to 10 , 000 . the terminology &# 34 ; 6 - substituted glucose unit &# 34 ; as used herein means an anhydrous glucose unit of cellulose having a substituent only at the 6 - position thereof . it is necessary that the starting cellulose ether having a first substituent should comprise at least 18 mol % of a 6 - substituted glucose unit and at least 15 mol % of an unsubstituted glucose unit . such a specific cellulose ether can be obtained by etherification in an increased alkali concentration , for a reduced mercerization time , and at an increased etherification temperature . more specifically , the alkali concentration in the aqueous phase is generally from 25 % to 50 % by weight and more preferably from 30 % to 35 % by weight , and the mercerization is carried out preferably for from 30 minutes to 4 hours , and more preferably from 30 minutes to 2 hours . examples of the alkali include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide . the subsequent etherification reaction is conducted preferably at a temperature of 50 ° c . or higher for a period of 150 minutes at the longest . while the upper limit of the temperature of the etherification depends on the boiling temperature of reaction solvents or reagents to be used , in the case of using an alcoholic slovent which is generally used in the present invention , the upper limit of the temperature is approximately from 70 ° to 80 ° c . the lower limit of the time for the etherification depends on reagents to be used or performance of objective products , but the lower limit is geneally about 30 minutes . in the case of hydroxyalkylation , the concentration of the alkali is preferably from 30 to 35 % by weight , the mercerization time is generally from 30 minutes to 120 minutes and preferably from 30 to 60 minutes , and the etherification is conducted at a possible high temperature , preferably from 50 ° to 80 ° c ., for 1 to 2 hours . the cellulose ether obtained under the above - described conditions has a chemical structure in which the substituent is distributed non - uniformly . it is unsuitable for general purposes but very favorable as a starting compound for preparing the mixed cellulose ether of the present invention . the process for introducing a second substituent , a dihydroxyalkyl group , into the starting cellulose ether is not limited , but special care should be taken in distribution of an alkali . that is , it is desired to conduct the second etherification in the presence of an alkali in an amount of not more than 0 . 5 mol , more preferably from 0 . 1 to 0 . 4 mol , per mol of glucose unit . examples of the alkali include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide . if the reaction is conducted at a high temperature at too a high alkali concentration , the dihydroxyalkyl group would form not only a chain but a network structure only to produce a mixed cellulose ether having seriously reduced solubility in water , etc . suitable dihydroxyalkylating agent to be used in the second etherification includes glycidyl type compounds such as glycidol and halohydrin type compounds such as 1 - chloro - 2 , 3 - dihydroxypropane . the present invention is now illustrated in greater detail with reference to examples , but it should be understood that the present invention is not construed as being limited thereto . all the percents are given by weight unless otherwise indicated . the substituent distribution of the cellulose ethers prepared in examples was determined as follows . a cellulose ether was hydrolyzed with 72 % sulfuric acid , and the hydrolyzate was reduced with sodium borohydride to open the glucose ring . in the case of alkyl ethers , the product as obtained was purified and subjected to gas chromatography . in the case of hydroxyalkyl ethers , the product is treated with sodium hydride to activate the hydroxyl groups and then methylated with methyl iodide , and the resulting product is subjected to gas chromatography . a sample solution is put into an outer tube , and the outer tube containing the sample solution is placed on a white paper on which 15 black lines each having a width of 1 mm are drawn in parallel such that the space between the lines being next to each other is 1 mm . when a person peeps into the sample solution through an inner tube from the upper position , the height from the bottom of the inner tube when it comes to be impossible to distinguish the lines by moving the inner tube up an down is measured . the outer tube is composed of a glass cylinder having a height of 350 mm , an inner diameter of 25 mm and a thickness of 2 mm and a good quality glass plate having a thickness of 2 mm joined to the bottom of the glass cylinder , and the inner tube is composed of a glass cylinder having a height of 300 mm , an inner diameter of 15 mm and a thickness of 2 mm and a good quality glass plate having a thickness of 2 mm joined to the bottom of the glass cylinder . clarity ( mm ) as used hereinafter means an average of the values obtained by repeating the procedure three times . the clarity of the resulting aqueous cellulose ether solutions measured in the following examples can be considered as an index of solubility in water of the resulting cellulose ethers . linters weighing 20 g having been ground in a cutting mill were put in a separable flask , and 253 g of t - butyl alcohol and 14 . 5 g of water were added thereto to make a slurry . a solution of 6 . 9 g of sodium hydroxide in 20 g of water was added to the slurry with stirring , and the stirring was continued for 2 hours for mercerization . after completion of the mercerization , 2 . 9 mols of ethylene oxide was added thereto per mol of anhydrous glucose unit , and the mixture was allowed to react at 60 ° c . for 1 hour and then at 75 ° c . for 1 hour . the resulting hydroxyethyl cellulose had a degree of hydroxyethyl substitution of 1 . 74 and contained 23 . 1 mol % of a 6 - substituted glucose unit and 31 . 4 mol % of an unsubstituted glucose unit . a 1 % aqueous solution of the product had a viscosity of 6000 cps ( measured at 25 ° c . and 30 rpm , hereinafter the same ) and a clarity of 30 mm , showing very poor solubility . the thus obtained hydroxyethyl cellulose was etherified with 2 . 0 mols of chloropropanediol per mol of anhydrous glucose unit at a 27 % alkali concentration of the water phase using sodium hydroxide . the resulting dihydroxypropylhydroxyethyl cellulose had a degree of dihydroxypropyl substitution of 1 . 20 . a 1 % aqueous solution thereof had a viscosity of 3500 cps and a clarity of 300 mm or more . the utilization of the etherifying reagent was 60 %. a commercially available hydroxyethyl cellulose ( hec ) was analyzed to find that the degree of hydroxyethyl substitution was 2 . 32 , the unsubstituted glucose unit content was 7 . 2 mol %, and the 6 - substituted glucose unit content was 17 . 5 mol % and that a 1 % aqueous solution thereof had a viscosity of 5000 cps and a clarity of 600 mm or more . hec was dihydroxypropylated in the same manner as in example 1 . the resulting mixed cellulose ether was found to have a degree of dihydroxypropyl substitution of 0 . 72 , and its 1 % aqueous solution had a viscosity of 180 cps and a clarity of 55 mm . the utilization of the etherifying reagent was 36 %. linters weighing 20 g having been ground in a cutting mill were dispersed in 256 g of t - butyl alcohol and 12 g of water to make a slurry . a solution of 6 . 9 g of sodium hydroxide in 20 g of water was added to the slurry with stirring , and the stirring was continued for 1 hour for mercerization . after completion of the mercerization , 2 . 1 mols of ethylene oxide was added thereto per mol of anhydrous glucose unit , and the mixture was allowed to react at 70 ° c . for 2 hours . the resulting hydroxyethyl cellulose had a degree of hydroxyethyl substitution of 1 . 30 and a 6 - substituted glucose unit content of 33 mol %, and an unsubstituted glucose unit content of 32 mol %. a 1 % aqueous solution of the product had a viscosity of 630 cps . the clarity of the aqueous solution was too poor to measure . the thus obtained hydroxyethyl cellulose was etherified with 2 . 0 mols of chloropropanediol per mol of anhydrous glucose unit at a 27 % alkali concentration of the water phase using sodium hydroxide . the resulting dihydroxypropylhydroxyethyl cellulose had a degree of dihydroxypropyl substitution of 1 . 12 . a 1 % aqueous solution thereof had a viscosity of 3000 cps and a clarity of 300 mm or more . the utilization of the etherifying reagent was 56 %. linters ground in the same manner as in example 1 were reacted with 3 . 14 mols of ethylene oxide per mol of anhydrous glucose unit in the similar reaction system as in example 1 at 60 ° c . for 80 minutes and then at 70 ° c . for 90 minutes . the resulting hydroxyethyl cellulose had a degree of hydroxyethyl substitution of 2 . 20 , a 6 - substituted glucose unit content of 16 mol %, and an unsubstituted glucose unit content of 14 mol %. a 1 % aqueous solution thereof had a viscosity of 4500 cps . the thus obtained hydroxyethyl cellulose was dihydroxypropylated in the same manner as in example 2 . the resulting dihydroxypropylhydroxyethyl cellulose had a degree of dihydroxypropyl substitution of 0 . 76 . a 1 % aqueous solution thereof had a viscosity of 3540 cps and a clarity of 300 mm or more . the utilization of the etherifying reagent was 38 %. as described and demonstrated above , to start with a cellulose ether having a specific distribution of a first substituent makes it possible to introduce a dihydroxyalkyl group as a second substituent at a high utilization of the etherifying reagent . an aqueous solution of the resulting mixed cellulose ether exhibits excellent clarity . while the invention has been described in detail and with reference to specific examples thereof , it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof .