W. Bell & Co., a catalogue showroom company headquartered in Rockville, reported after-tax earnings of $1.58 million (84 cents a share) for the 26-week period ended Dec. 26 compared with earnings of $1.94 million ($1.03) a year before.

Sales in that period increased to $80.05 million, a 33 percent gain over sales of $60.35 million the previous year.

Company officials said the decrease in earnings occurred because increased sales promotions in the pre-Christmas selling season reduced gross margins and because of the costs of opening two locations in Chicago, a new market for the firm.

Earnings for the 13 weeks ended Dec. 26 were $1.46 million (77 cents) compared with $1.85 million (98 cents) in the same period a year before. Sales for that 13-week period were $56.27 million compared with $44.68 million in 1980.

The per-share figures reflect a 50 percent stock dividend paid in December 1981 and a 20 percent stock dividend paid in December 1980.

Radiation Systems Inc. of McLean reported net income for its second quarter ended December 31 of $563,000 (29 cents) compared with $199,000 (17 cents) for the same quarter a year ago. Sales were $2.981 million compared with sales of $2.421 million a year before.

For the six months ended Dec. 31, net income was $1.03 million (54 cents) on sales of $5.54 million compared with $379,000 (32 cents) on sales of $4.18 million for the year before.